feat: prune orphaned photo rows + retry-pending action

Adds /api/v1/library/maintenance/{missing-stats,prune-missing} backed
by a new cleanup helper that deletes Photo rows whose files no longer
exist on disk under a *mounted* source root. Skips photos under
unmounted roots so a temporarily-disconnected drive doesn't get
silently nuked.

Settings panel surfaces the orphan count with a destructive Prune
button, plus a "Kick pending" action that re-queues photos stuck in
processing_status='pending' (typically left behind when the scanner
created the row but the worker never picked up the thumbnail task).

Common trigger: PHOTO_DIRS in .env was repointed at a different
library root, leaving every old row dangling.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/routers/library.py

@@ -142,6 +142,12 @@ class RegenerateThumbnailsRequest(BaseModel):
         default=False,
         description="If true, only re-queue photos whose processing_status is 'failed'.",
     )
+    only_pending: bool = Field(
+        default=False,
+        description="If true, only (re-)queue photos whose processing_status is 'pending'. "
+                    "Useful for kicking rows that were created by a scan but never had "
+                    "their thumbnail task picked up.",
+    )
 
 
 @router.get("/maintenance/thumbnail-stats")
@@ -207,6 +213,8 @@ async def regenerate_thumbnails(
         query = query.where(Photo.media_type.in_(media_types))
     if body.only_failed:
         query = query.where(Photo.processing_status == 'failed')
+    if body.only_pending:
+        query = query.where(Photo.processing_status == 'pending')
 
     photos = (await db.execute(query)).scalars().all()
 
@@ -390,6 +398,29 @@ async def get_worker_status(db: AsyncSession = Depends(get_db)):
     }
 
 
+@router.get("/maintenance/missing-stats")
+async def get_missing_stats():
+    """Count photos whose files no longer exist on disk under a mounted
+    source root. Surfaced in Settings so the user can see a number before
+    pulling the trigger on prune-missing. Cheap enough to call freely."""
+    from app.services.cleanup import prune_missing_photos
+    return await prune_missing_photos(dry_run=True)
+
+
+@router.post("/maintenance/prune-missing")
+async def run_prune_missing():
+    """Actually delete the orphaned photo rows reported by /missing-stats.
+    Common cause: PHOTO_DIRS in .env was repointed at a different library
+    leaving every old row dangling. Skips any photo whose source root
+    isn't currently mounted (almost always means an unmounted drive)."""
+    from app.services.cleanup import prune_missing_photos
+    try:
+        return {"status": "success", **(await prune_missing_photos(dry_run=False))}
+    except Exception as e:
+        logger.error(f"Prune missing failed: {e}")
+        return {"status": "error", "message": str(e)}
+
+
 @router.post("/maintenance/cleanup")
 async def run_data_integrity_cleanup():
     """Re-run the source-roots / folders / photos data-integrity cleanup