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/tests/integration/test_upload.py

@@ -3,10 +3,10 @@ T026 — valid JPEG upload → 201, record in DB, object in MinIO
 T027 — same image uploaded twice → 200, duplicate: true, no second MinIO object
 T028 — invalid MIME type → 422 invalid_mime_type (error envelope with code field)
 T029 — file > MAX_UPLOAD_BYTES → 422 file_too_large
-T079 — GET /api/v1/images/{id} 404 → error envelope shape
+T013 — upload produces short_id; storage_key equals short_id; thumbnail_key = {short_id}-thumb
 """
 import io
-import uuid
+import re
 from unittest.mock import patch
 
 import pytest
@@ -111,13 +111,81 @@ async def test_upload_oversized_file_returns_422(authed_client):
 
 @pytest.mark.asyncio
 async def test_get_unknown_image_returns_404_with_envelope(client):
-    response = await client.get(f"/api/v1/images/{uuid.uuid4()}")
+    response = await client.get("/api/v1/i/NotFound")
     assert response.status_code == 404
     body = response.json()
     assert body["code"] == "image_not_found"
     assert "detail" in body
 
 
+_SHORT_ID_RE = re.compile(r"^[a-zA-Z0-9]{8}$")
+
+
+@pytest.mark.asyncio
+async def test_upload_returns_short_id(authed_client):
+    client, token = authed_client
+    data = _minimal_jpeg()
+    response = await client.post(
+        "/api/v1/images",
+        files={"file": ("s1.jpg", io.BytesIO(data), "image/jpeg")},
+        headers={"Authorization": f"Bearer {token}"},
+    )
+    assert response.status_code == 201
+    body = response.json()
+    assert "short_id" in body
+    assert _SHORT_ID_RE.match(body["short_id"]), f"short_id invalid: {body['short_id']}"
+
+
+@pytest.mark.asyncio
+async def test_upload_storage_key_equals_short_id(authed_client):
+    client, token = authed_client
+    data = _real_jpeg(color=(10, 20, 30))
+    response = await client.post(
+        "/api/v1/images",
+        files={"file": ("s2.jpg", io.BytesIO(data), "image/jpeg")},
+        headers={"Authorization": f"Bearer {token}"},
+    )
+    assert response.status_code == 201
+    body = response.json()
+    assert body["storage_key"] == body["short_id"]
+
+
+@pytest.mark.asyncio
+async def test_upload_thumbnail_key_equals_short_id_thumb(authed_client):
+    client, token = authed_client
+    data = _real_jpeg(color=(30, 60, 90))
+    response = await client.post(
+        "/api/v1/images",
+        files={"file": ("s3.jpg", io.BytesIO(data), "image/jpeg")},
+        headers={"Authorization": f"Bearer {token}"},
+    )
+    assert response.status_code == 201
+    body = response.json()
+    if body["thumbnail_key"] is not None:
+        assert body["thumbnail_key"] == f"{body['short_id']}-thumb"
+
+
+@pytest.mark.asyncio
+async def test_duplicate_upload_returns_same_short_id(authed_client):
+    client, token = authed_client
+    data = _real_jpeg(color=(200, 100, 50))
+    headers = {"Authorization": f"Bearer {token}"}
+    r1 = await client.post(
+        "/api/v1/images",
+        files={"file": ("dup_short.jpg", io.BytesIO(data), "image/jpeg")},
+        headers=headers,
+    )
+    assert r1.status_code in (200, 201)
+    r2 = await client.post(
+        "/api/v1/images",
+        files={"file": ("dup_short.jpg", io.BytesIO(data), "image/jpeg")},
+        headers=headers,
+    )
+    assert r2.status_code == 200
+    assert r2.json()["duplicate"] is True
+    assert r2.json()["short_id"] == r1.json()["short_id"]
+
+
 @pytest.mark.asyncio
 async def test_upload_returns_thumbnail_key(authed_client):
     client, token = authed_client
@@ -133,9 +201,9 @@ async def test_upload_returns_thumbnail_key(authed_client):
     assert body["thumbnail_key"] is not None
     assert body["thumbnail_key"].endswith("-thumb")
     assert "file_url" in body
-    assert body["file_url"].startswith("/api/v1/images/")
+    assert body["file_url"].startswith("/api/v1/i/")
     assert "thumbnail_url" in body
-    assert body["thumbnail_url"].startswith("/api/v1/images/")
+    assert body["thumbnail_url"].startswith("/api/v1/i/")
 
 
 @pytest.mark.asyncio
@@ -177,5 +245,5 @@ async def test_upload_succeeds_when_thumbnail_fails(authed_client):
     body = response.json()
     assert body["thumbnail_key"] is None
     assert "file_url" in body
-    assert body["file_url"].startswith("/api/v1/images/")
+    assert body["file_url"].startswith("/api/v1/i/")
     assert body["thumbnail_url"] is None