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/routers/api_frames.py

@@ -28,7 +28,7 @@ from sqlalchemy.orm import Session
 from .. import gitea_releases, photo_queue, quiet_hours
 from ..auth import require_frame_control, require_frame_view, require_user_api
 from ..db import frame_locked, get_db
-from ..image_pipeline import PALETTE_LABELS, hex_to_rgb
+from ..image_pipeline import DEFAULT_DISPLAY_MODE, DISPLAY_MODES, PALETTE_LABELS, hex_to_rgb
 from ..firmware import firmware_path, parse_app_version
 from ..models import BatteryLog, Frame
 from .common import (
@@ -70,7 +70,7 @@ def api_config_save(
     album_id: str | None = Form(None),
     order: str | None = Form(None),
     refresh_interval_s: int | None = Form(None),
-    smart_crop_faces: bool | None = Form(None),
+    display_mode: str | None = Form(None),
     queue_target_len: int | None = Form(None),
     orientation: str | None = Form(None),
     quiet_hours_enabled: bool | None = Form(None),
@@ -104,8 +104,8 @@ def api_config_save(
             cfg.refresh_interval_s = max(
                 MIN_REFRESH_INTERVAL_S, min(MAX_REFRESH_INTERVAL_S, refresh_interval_s)
             )
-        if smart_crop_faces is not None:
-            cfg.smart_crop_faces = smart_crop_faces
+        if display_mode is not None:
+            cfg.display_mode = display_mode if display_mode in DISPLAY_MODES else DEFAULT_DISPLAY_MODE
         if queue_target_len is not None:
             cfg.queue_target_len = max(MIN_QUEUE_TARGET_LEN, min(MAX_QUEUE_TARGET_LEN, queue_target_len))
         if orientation is not None: