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/app/static/theme.css

@@ -197,6 +197,35 @@ details.card .sub { margin-top: 8px; }
   width: 58px;
 }
 
+.slider-value {
+  float: right;
+  font-weight: 400;
+  color: var(--text-muted);
+  font-variant-numeric: tabular-nums;
+}
+input[type="range"] {
+  width: 100%;
+  margin-top: 8px;
+  accent-color: var(--accent);
+  padding: 0;
+  background: none;
+}
+
+.preview-compare {
+  display: grid;
+  grid-template-columns: repeat(auto-fit, minmax(200px, 1fr));
+  gap: 14px;
+  margin-top: 14px;
+}
+.preview-img {
+  width: 100%;
+  display: block;
+  border-radius: 8px;
+  border: 1px solid var(--border);
+  background: var(--surface-alt);
+  min-height: 100px;
+}
+
 .card {
   background: var(--surface);
   border: 1px solid var(--border);