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/library/library.component.ts

@@ -70,8 +70,8 @@ const PLACEHOLDER_SVG = `data:image/svg+xml,<svg xmlns="http://www.w3.org/2000/s
           class="image-card"
           role="button"
           tabindex="0"
-          (click)="router.navigate(['/images', img.id])"
-          (keydown.enter)="router.navigate(['/images', img.id])"
+          (click)="router.navigate(['/i', img.short_id])"
+          (keydown.enter)="router.navigate(['/i', img.short_id])"
         >
           <img
             [src]="img.thumbnail_url ?? img.file_url"