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>

ui/src/app/services/image.service.ts

@@ -4,6 +4,7 @@ import { Observable } from 'rxjs';
 
 export interface ImageRecord {
   id: string;
+  short_id: string;
   hash: string;
   filename: string;
   mime_type: string;
@@ -50,14 +51,14 @@ export class ImageService {
   }
 
   get(id: string): Observable<ImageRecord> {
-    return this.http.get<ImageRecord>(`${this.base}/images/${id}`);
+    return this.http.get<ImageRecord>(`${this.base}/i/${id}`);
   }
 
   updateTags(id: string, tags: string[]): Observable<ImageRecord> {
-    return this.http.patch<ImageRecord>(`${this.base}/images/${id}/tags`, { tags });
+    return this.http.patch<ImageRecord>(`${this.base}/i/${id}/tags`, { tags });
   }
 
   delete(id: string): Observable<void> {
-    return this.http.delete<void>(`${this.base}/images/${id}`);
+    return this.http.delete<void>(`${this.base}/i/${id}`);
   }
 }