Display mode: crop to fill, crop to faces, stretch to fill, shrink to fit

Replaces the smart_crop_faces boolean with a 4-way display_mode select
on each frame's Configuration tab (image_pipeline.DISPLAY_MODES):

- Crop to fill / Crop to faces: the previous False/True behavior,
  unchanged (center-crop trimming excess, optionally shifted to keep
  faces on screen).
- Stretch to fill (new): fills the panel exactly, aspect ratio not
  preserved -- a plain resize, no crop.
- Shrink to fit (new): the whole photo visible, letterboxed with white
  where it doesn't fill the panel.

Named-face overlay label positioning (face_labels.py, the manage menu's
"who's in this photo") now goes through a shared _placement_transform()
in image_pipeline.py instead of duplicating crop-box math, so label
placement stays correct (and in-bounds) under all four modes, not just
the two crop ones -- letterbox/stretch never crop a face out, so labels
just use straight scale+offset math there.

Schema migration v5 adds display_mode, backfills it from the old
boolean (True/False -> crop_faces/crop_fill), and drops the boolean.

Verified against the live-shaped test database: the migration
(existing frames correctly preserved as crop_faces), the config page's
new 4-option select, actual renders under letterbox (confirmed real
white letterbox padding in the packed panel-code bytes) and
stretch_fill, invalid-input fallback, and the standing legacy-device
curl suite.

server/app/migration.py

@@ -59,11 +59,23 @@ def _migration_4(conn) -> None:
     conn.execute(text("ALTER TABLE frames ADD COLUMN palette_rgb TEXT"))
 
 
+def _migration_5(conn) -> None:
+    """Replaces the smart_crop_faces boolean with display_mode (see
+    image_pipeline.DISPLAY_MODES) -- crop_faces/crop_fill are exactly
+    the old True/False behavior, stretch_fill/letterbox are new."""
+    conn.execute(text("ALTER TABLE frames ADD COLUMN display_mode TEXT NOT NULL DEFAULT 'crop_faces'"))
+    conn.execute(text(
+        "UPDATE frames SET display_mode = CASE WHEN smart_crop_faces THEN 'crop_faces' ELSE 'crop_fill' END"
+    ))
+    conn.execute(text("ALTER TABLE frames DROP COLUMN smart_crop_faces"))
+
+
 MIGRATIONS = [
     (1, _migration_1),
     (2, _migration_2),
     (3, _migration_3),
     (4, _migration_4),
+    (5, _migration_5),
 ]
 
 
@@ -126,7 +138,7 @@ def _ensure_frame_one() -> None:
             quiet_hours_start=cfg.quiet_hours_start,
             quiet_hours_end=cfg.quiet_hours_end,
             timezone=cfg.timezone,
-            smart_crop_faces=cfg.smart_crop_faces,
+            display_mode="crop_faces" if cfg.smart_crop_faces else "crop_fill",
             orientation=cfg.orientation,
             queue_target_len=cfg.queue_target_len,
             current_asset_id=cfg.current_asset_id,