Color/contrast/dithering sliders + before/after render preview

Advanced configuration gains three sliders (PIL ImageEnhance factors
for color/contrast, 0-2, 1=unchanged; a 0-1 dithering strength) applied
to every photo this frame renders. Confirmed the parameter conventions
against a similar project (jwchen119/EPF: ImageEnhance.Color/Contrast,
1.0 baseline) before implementing; dithering strength isn't natively
exposed by PIL's quantize(), so it's implemented by blending the source
toward its own flat/undithered quantization before running Floyd-
Steinberg on the blend -- at 0 there's no quantization error left to
diffuse (exactly the flat result), at 1 it's the original unmodified
behavior, with a smooth continuum between rather than dithering being
an on/off toggle.

image_pipeline.py split into composition (_compose), enhancement
(_enhance), quantization (_quantize), and transpose+pack stages so
render_frame (device bytes) and the new render_preview_png (a normal
viewable PNG, upright logical orientation) share the same pipeline
instead of duplicating it. Named-face overlay label math (face_labels.py)
was already routed through the shared _placement_transform, so it
needed no changes for the new params.

Also added the requested before/after comparison: the Configuration
tab's new Preview card shows the current photo's untouched Immich
preview next to that same photo run through the frame's actual saved
rendering pipeline (two new GET endpoints, /preview/original and
/preview/rendered) -- immediate visual feedback for tuning the palette
and these new sliders. "Refresh preview" re-fetches after saving.

Schema migration v6 adds color_boost/contrast_boost/dither_strength,
defaulting to 1.0/1.0/1.0 -- reproduces the exact previous rendering
until a frame's Configuration tab changes one.

Verified against the live-shaped test database: the migration, sliders
persisting and clamping out-of-range input, both preview endpoints
(real JPEG passthrough / real PNG at correct logical size+orientation),
confirmed dither_strength=0 actually changes the rendered bytes vs.
default, and the standing legacy-device curl suite.

server/README.md

@@ -147,16 +147,26 @@ Pages: `/` (routing hub), `/setup`, `/login`, `/claim`, `/settings`,
 - `GET  .../albums` -- the owner's Immich albums.
 - `POST .../config` -- **partial** update: only provided fields change
   (`name`, `album_id` -- resets queue/history on change --, `order`,
-  `refresh_interval_s`, `smart_crop_faces`, `queue_target_len`,
-  `orientation` (composed logically then rotated server-side; the
-  on-device manage overlay still renders native, a known limitation),
-  `quiet_hours_*` + `timezone` (a pure server-side decision shaping
-  what `refresh_interval_s` gets handed to the device),
-  `firmware_update_repo_url`, `firmware_auto_update`,
+  `refresh_interval_s`, `display_mode` (`crop_fill`/`crop_faces`/
+  `stretch_fill`/`letterbox`, see `image_pipeline.DISPLAY_MODES`),
+  `queue_target_len`, `orientation` (composed logically then rotated
+  server-side; the on-device manage overlay still renders native, a
+  known limitation), `quiet_hours_*` + `timezone` (a pure server-side
+  decision shaping what `refresh_interval_s` gets handed to the
+  device), `firmware_update_repo_url`, `firmware_auto_update`,
   `battery_alert_threshold_pct` -- percent, or `-1`/blank to disable --,
   `palette` -- exactly 6 `#rrggbb` values in black/white/yellow/red/
   blue/green order --, `palette_reset` -- `true` clears back to the
-  default palette).
+  default palette --, `color_boost`/`contrast_boost` -- PIL
+  `ImageEnhance` factors, 0-2, 1 = unchanged --, `dither_strength` --
+  0-1, blends toward a flat/undithered quantization before running
+  Floyd-Steinberg, so 0 = no dithering texture and 1 = full strength).
+- `GET  .../preview/original`, `GET .../preview/rendered` -- the
+  before/after comparison on the Configuration tab: the current
+  photo's Immich preview untouched (JPEG), and that same photo run
+  through this frame's actual saved rendering pipeline (PNG, upright
+  logical orientation, not packed device bytes) -- reflects saved
+  settings, not unsaved slider positions.
 - `POST .../take-control` -- always succeeds for a linked user.
 - `GET  .../stats`, `GET .../battery-log`, `GET .../thumbnail/{asset_id}`.
 - `POST .../firmware` (manual .bin upload, esp_app_desc_t-validated),