fix(photos): accept bare-date filter bounds, send T00:00:00 from UI

GET /api/v1/photos rejected ?date_from=2026-04-10 with 422 because
pydantic v2's datetime parser doesn't accept date-only strings. The
frontend has been padding date_to with T23:59:59 forever to make the
upper bound inclusive, but date_from went out as a bare YYYY-MM-DD,
so every date-range filter request 422'd and the grid showed nothing.

Frontend: pad date_from with T00:00:00 the same way date_to gets
T23:59:59 — symmetry, and pydantic v2 accepts the full form.

Backend: change date_from/date_to to Optional[str] and parse with
datetime.fromisoformat in the handler. fromisoformat accepts both
bare dates ('2026-04-10' -> midnight) and full ISO strings, so any
older client that still sends a date-only value continues to work.
Tz-aware values get coerced to naive UTC before binding (matches the
taken_at column's  shape and the same
fix applied to PATCH /photos/{id} earlier today). Bad input returns
400 with a clear message instead of pydantic's 422.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/routers/photos.py

@@ -52,8 +52,12 @@ router = APIRouter()
 @router.get("")
 async def list_photos(
     q: Optional[str] = None,
-    date_from: Optional[datetime] = None,
-    date_to: Optional[datetime] = None,
+    # Accept either a bare date ("2026-04-10") or a full ISO datetime
+    # ("2026-04-10T23:59:59"). pydantic v2's datetime parser rejects
+    # the bare form with 422; we coerce manually below so older
+    # clients that send a date-only string keep working.
+    date_from: Optional[str] = None,
+    date_to: Optional[str] = None,
     folder_id: Optional[str] = None,
     tag_ids: Optional[str] = None,
     media_type: Optional[str] = None,
@@ -129,11 +133,33 @@ async def list_photos(
             )
         )
     
-    # Date range
-    if date_from:
-        filters.append(Photo.taken_at >= date_from)
-    if date_to:
-        filters.append(Photo.taken_at <= date_to)
+    # Date range. Both inputs are strings to keep pydantic from rejecting
+    # bare-date forms ("2026-04-10") with 422; coerce here. fromisoformat
+    # accepts both bare dates and full ISO datetimes — when given a date,
+    # it returns midnight, which is what we want for the lower bound.
+    def _parse_bound(s: Optional[str]) -> Optional[datetime]:
+        if not s:
+            return None
+        try:
+            dt = datetime.fromisoformat(s)
+        except ValueError:
+            raise HTTPException(
+                status_code=400,
+                detail=f"Invalid date: {s!r} (expected YYYY-MM-DD or ISO 8601)",
+            )
+        # If the caller sent a tz-aware string, normalize to naive UTC —
+        # the photos.taken_at column is `timestamp without time zone`.
+        if dt.tzinfo is not None:
+            from datetime import timezone
+            dt = dt.astimezone(timezone.utc).replace(tzinfo=None)
+        return dt
+
+    parsed_date_from = _parse_bound(date_from)
+    parsed_date_to = _parse_bound(date_to)
+    if parsed_date_from:
+        filters.append(Photo.taken_at >= parsed_date_from)
+    if parsed_date_to:
+        filters.append(Photo.taken_at <= parsed_date_to)
     
     # Folder filter. The sidebar can pass either a SourceRoot id or a
     # Folder id; both should include descendants so clicking a parent