fix: stop scan crashing on duplicate file hashes

scan.py used scalar_one_or_none() to test whether any other photo
shared the same file_hash, but that helper raises MultipleResultsFound
the moment 2+ rows match — i.e. exactly the duplicate case it was
trying to flag. Every file beyond the second copy bombed out with
"Multiple rows were found when one or none was required" and was
left in the failed bucket. Replace with a COUNT(*) > 0 check.

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

backend/app/tasks/scan.py

@@ -11,7 +11,7 @@ import json
 from typing import List, Dict, Optional
 
 from celery import shared_task
-from sqlalchemy import select
+from sqlalchemy import select, func
 from sqlalchemy.ext.asyncio import AsyncSession
 import aiofiles
 import redis
@@ -172,10 +172,21 @@ async def _scan_folder_async(folder_path: str, source_root_id: Optional[str], ta
                             # Calculate file hash for duplicate detection
                             file_hash = await calculate_file_hash(filepath)
 
-                            # Check for duplicate by hash
-                            duplicate = await session.execute(
-                                select(Photo).where(Photo.file_hash == file_hash)
-                            ) if file_hash else None
+                            # Check for duplicate by hash. We only care
+                            # whether *any* other photo shares this hash, so
+                            # use a count rather than scalar_one_or_none()
+                            # which raises "Multiple rows were found" the
+                            # moment the library has 2+ copies of the same
+                            # file (i.e. exactly the case we're trying to
+                            # flag).
+                            is_dup = False
+                            if file_hash:
+                                dup_count = (await session.execute(
+                                    select(func.count(Photo.id)).where(
+                                        Photo.file_hash == file_hash
+                                    )
+                                )).scalar() or 0
+                                is_dup = dup_count > 0
 
                             # Create photo entry
                             photo = Photo(
@@ -188,7 +199,7 @@ async def _scan_folder_async(folder_path: str, source_root_id: Optional[str], ta
                                 file_size=stat.st_size,
                                 taken_at=datetime.fromtimestamp(stat.st_mtime),
                                 taken_at_source='filesystem',
-                                is_duplicate=bool(duplicate.scalar_one_or_none() if duplicate else False),
+                                is_duplicate=is_dup,
                                 processing_status='pending'
                             )