Feat: Replace UUID image identifiers with 8-character base62 short IDs

Short IDs become the canonical identifier in URLs (/i/:short_id),
MinIO/R2 storage keys, and all API responses. Hash-based deduplication
is preserved. Includes two-phase Alembic migration (003 adds nullable
column, 004 enforces NOT NULL) with a backfill script to copy storage
objects and populate short_id for existing images.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

api/app/routers/images.py

@@ -1,7 +1,7 @@
 import asyncio
 import logging
+import re
 import struct
-import uuid
 from typing import Any
 
 from fastapi import APIRouter, Depends, File, Form, HTTPException, Response, UploadFile
@@ -15,7 +15,7 @@ from app.repositories.image_repo import ImageRepository
 from app.repositories.tag_repo import TagRepository
 from app.storage.backend import StorageBackend
 from app.thumbnail import generate_thumbnail
-from app.utils import compute_sha256
+from app.utils import compute_sha256, generate_short_id
 from app.validation import FileSizeError, MimeTypeError, validate_file_size, validate_mime_type
 
 logger = logging.getLogger(__name__)
@@ -23,22 +23,35 @@ logger = logging.getLogger(__name__)
 router = APIRouter(tags=["images"])
 
 
+_SHORT_ID_RE = re.compile(r"^[a-zA-Z0-9]{8}$")
+
+
 def _error(detail: str, code: str, status: int):
     raise HTTPException(status_code=status, detail={"detail": detail, "code": code})
 
 
+def _validate_short_id(short_id: str) -> str:
+    if not _SHORT_ID_RE.match(short_id):
+        raise HTTPException(
+            status_code=422,
+            detail={"detail": "Invalid image ID", "code": "invalid_short_id"},
+        )
+    return short_id
+
+
 def _image_to_dict(
     image: Image, *, cdn_base: str | None = None, duplicate: bool | None = None
 ) -> dict[str, Any]:
     _base = cdn_base.strip().rstrip("/") if cdn_base else None
-    file_url = f"{_base}/{image.storage_key}" if _base else f"/api/v1/images/{image.id}/file"
+    file_url = f"{_base}/{image.storage_key}" if _base else f"/api/v1/i/{image.short_id}/file"
     thumbnail_url = (
-        (f"{_base}/{image.thumbnail_key}" if _base else f"/api/v1/images/{image.id}/thumbnail")
+        (f"{_base}/{image.thumbnail_key}" if _base else f"/api/v1/i/{image.short_id}/thumbnail")
         if image.thumbnail_key
         else None
     )
     data: dict[str, Any] = {
         "id": str(image.id),
+        "short_id": image.short_id,
         "hash": image.hash,
         "filename": image.filename,
         "mime_type": image.mime_type,
@@ -169,29 +182,49 @@ async def upload_image(
             )
 
     width, height = _read_image_dimensions(data, mime_type)
-    await storage.put(hash_hex, data, mime_type)
 
-    thumbnail_key: str | None = None
-    try:
-        thumb_bytes = await asyncio.to_thread(generate_thumbnail, data, mime_type)
-        await storage.put(f"{hash_hex}-thumb", thumb_bytes, "image/webp")
-        thumbnail_key = f"{hash_hex}-thumb"
-    except Exception:
-        logger.warning(
-            "Thumbnail generation failed for %s; upload will proceed without thumbnail", hash_hex
+    from sqlalchemy.exc import IntegrityError
+
+    for _ in range(10):
+        short_id = generate_short_id()
+        await storage.put(short_id, data, mime_type)
+
+        thumbnail_key: str | None = None
+        try:
+            thumb_bytes = await asyncio.to_thread(generate_thumbnail, data, mime_type)
+            await storage.put(f"{short_id}-thumb", thumb_bytes, "image/webp")
+            thumbnail_key = f"{short_id}-thumb"
+        except Exception:
+            logger.warning(
+                "Thumbnail generation failed for %s; proceeding without thumbnail", short_id
+            )
+
+        try:
+            image = await image_repo.create(
+                hash_hex=hash_hex,
+                filename=file.filename or "upload",
+                mime_type=mime_type,
+                size_bytes=len(data),
+                width=width,
+                height=height,
+                storage_key=short_id,
+                short_id=short_id,
+                thumbnail_key=thumbnail_key,
+            )
+            break
+        except IntegrityError:
+            await db.rollback()
+            await storage.delete(short_id)
+            if thumbnail_key:
+                await storage.delete(thumbnail_key)
+            thumbnail_key = None
+            continue
+    else:
+        raise HTTPException(
+            status_code=500,
+            detail={"detail": "Failed to assign unique ID", "code": "id_collision"},
         )
 
-    image = await image_repo.create(
-        hash_hex=hash_hex,
-        filename=file.filename or "upload",
-        mime_type=mime_type,
-        size_bytes=len(data),
-        width=width,
-        height=height,
-        storage_key=hash_hex,
-        thumbnail_key=thumbnail_key,
-    )
-
     if tag_names:
         tag_repo = TagRepository(db)
         await tag_repo.attach_tags(image, tag_names)
@@ -221,15 +254,16 @@ async def list_images(
     }
 
 
-@router.get("/images/{image_id}")
+@router.get("/i/{short_id}")
 async def get_image(
-    image_id: uuid.UUID,
+    short_id: str,
     db: AsyncSession = Depends(get_db),
     settings=Depends(get_settings),
 ):
+    _validate_short_id(short_id)
     _cdn_base = settings.s3_public_base_url
     image_repo = ImageRepository(db)
-    image = await image_repo.get_by_id(image_id)
+    image = await image_repo.get_by_short_id(short_id)
     if not image:
         raise HTTPException(
             status_code=404,
@@ -238,14 +272,15 @@ async def get_image(
     return _image_to_dict(image, cdn_base=_cdn_base)
 
 
-@router.get("/images/{image_id}/file")
+@router.get("/i/{short_id}/file")
 async def serve_image_file(
-    image_id: uuid.UUID,
+    short_id: str,
     db: AsyncSession = Depends(get_db),
     storage: StorageBackend = Depends(get_storage),
 ):
+    _validate_short_id(short_id)
     image_repo = ImageRepository(db)
-    image = await image_repo.get_by_id(image_id)
+    image = await image_repo.get_by_short_id(short_id)
     if not image:
         raise HTTPException(
             status_code=404,
@@ -268,14 +303,15 @@ async def serve_image_file(
     )
 
 
-@router.get("/images/{image_id}/thumbnail")
+@router.get("/i/{short_id}/thumbnail")
 async def serve_image_thumbnail(
-    image_id: uuid.UUID,
+    short_id: str,
     db: AsyncSession = Depends(get_db),
     storage: StorageBackend = Depends(get_storage),
 ):
+    _validate_short_id(short_id)
     image_repo = ImageRepository(db)
-    image = await image_repo.get_by_id(image_id)
+    image = await image_repo.get_by_short_id(short_id)
     if not image:
         raise HTTPException(
             status_code=404,
@@ -300,17 +336,18 @@ async def serve_image_thumbnail(
     )
 
 
-@router.patch("/images/{image_id}/tags")
+@router.patch("/i/{short_id}/tags")
 async def update_image_tags(
-    image_id: uuid.UUID,
+    short_id: str,
     body: dict,
     db: AsyncSession = Depends(get_db),
     _: Identity = Depends(require_auth),
     settings=Depends(get_settings),
 ):
+    _validate_short_id(short_id)
     _cdn_base = settings.s3_public_base_url
     image_repo = ImageRepository(db)
-    image = await image_repo.get_by_id(image_id)
+    image = await image_repo.get_by_short_id(short_id)
     if not image:
         raise HTTPException(
             status_code=404,
@@ -332,15 +369,16 @@ async def update_image_tags(
     return _image_to_dict(image, cdn_base=_cdn_base)
 
 
-@router.delete("/images/{image_id}", status_code=204)
+@router.delete("/i/{short_id}", status_code=204)
 async def delete_image(
-    image_id: uuid.UUID,
+    short_id: str,
     db: AsyncSession = Depends(get_db),
     storage: StorageBackend = Depends(get_storage),
     _: Identity = Depends(require_auth),
 ):
+    _validate_short_id(short_id)
     image_repo = ImageRepository(db)
-    image = await image_repo.get_by_id(image_id)
+    image = await image_repo.get_by_short_id(short_id)
     if not image:
         raise HTTPException(
             status_code=404,