refactor: merge reject into trash as a single soft-trash concept

The Photo model previously had two near-identical "negative culling"
states: is_rejected (a flag) and is_trashed (a flag plus a file move).
Lightroom users typically use one or the other, never both, and the
file-move semantics of the old trash made it harder to undo. Merging
into a single soft is_trashed flag — file stays on disk, restore is a
flag flip, permanent deletion still happens via DELETE /trash/empty.

Backend
- Drop is_rejected from PhotoBase, PhotoResponse, PhotoUpdate, the
  list endpoint filter, and the bulk-action 'reject' branch.
- Add is_trashed to PhotoUpdate so the PATCH path can set it.
- Drop is_rejected Column declaration from the SQLAlchemy model. The
  legacy DB column may persist on existing installs but is no longer
  read or written; SQLAlchemy ignores extra columns.
- Rewrite DELETE /photos/{id} as a soft trash: just sets is_trashed=
  true and trashed_at=now, no shutil.move. Permanent deletion still
  goes through the trash router.

Frontend
- Photo TS type drops is_rejected, gains is_trashed.
- X keyboard shortcut now sets is_trashed=true (was is_rejected); U
  clears both is_picked and is_trashed.
- RightSidebar Reject button → Trash button (Trash2 icon).
- PhotoThumbnail flag overlay shows Trash2 icon for trashed photos
  instead of an X for rejected.
- KeyboardHints relabels X from "Reject" to "Trash".
- filterStore FlagFilter renames 'rejected' → 'trashed'; the params
  builder now sends is_trashed=true for the trashed filter (the list
  endpoint defaults to hiding trashed photos otherwise).
- FilterBar dropdown / URL sync allow-list updated accordingly.

No data migration: existing rejected photos remain as-is (flag stale)
and effectively become unflagged in the new model. Re-trash from the
UI to bring them into the new state.

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

backend/app/schemas/photos.py

@@ -20,7 +20,6 @@ class PhotoBase(BaseModel):
     rating: int = 0
     color_label: Optional[str] = None
     is_picked: bool = False
-    is_rejected: bool = False
 
 class PhotoResponse(PhotoBase):
     """Photo response schema"""
@@ -53,7 +52,7 @@ class PhotoUpdate(BaseModel):
     rating: Optional[int] = Field(None, ge=0, le=5)
     color_label: Optional[str] = None
     is_picked: Optional[bool] = None
-    is_rejected: Optional[bool] = None
+    is_trashed: Optional[bool] = None
     taken_at: Optional[datetime] = None
 
 class PhotoListResponse(BaseModel):