feat: overhaul remove-background with effects pipeline, consolidate color tools

Remove Background:
- Two-phase flow: AI removes bg once, then effects adjust instantly
- Blur background effect with real-time CSS preview (portrait mode)
- Drop shadow effect with opacity control
- Gradient backgrounds with presets, custom colors, and angle
- Custom background image upload (including HEIC/HEIF)
- Solid color backgrounds moved from Python to Node.js/Sharp
- Effects-only API endpoint for instant re-renders without AI re-run
- HEIC/HEIF input support (decoded before passing to Python/rembg)
- Passport/ID photo checkbox defaults ON for People subject
- Before/after slider preserved when no effects active
- 15 comprehensive Playwright e2e tests

Color Tools:
- Consolidated 4 tools (brightness-contrast, saturation, color-channels,
  color-effects) into single "Adjust Colors" tool
- Added exposure, temperature, tint, hue, sharpness controls
- SVG filter-based live preview for all adjustments
- Backward-compatible URL redirects from old tool paths

Other fixes:
- Favicon tool: download button instead of auto-download
- Batch processing: HEIC filename extension fix
- File store: processedFilename field for proper batch downloads

apps/web/src/components/tools/pipeline-step-settings.tsx

@@ -15,12 +15,7 @@ import { TextOverlayControls } from "./text-overlay-settings";
 import { UpscaleControls } from "./upscale-settings";
 import { WatermarkTextControls } from "./watermark-text-settings";
 
-const COLOR_TOOL_IDS = new Set([
-  "brightness-contrast",
-  "saturation",
-  "color-channels",
-  "color-effects",
-]);
+const COLOR_TOOL_IDS = new Set(["adjust-colors"]);
 
 interface PipelineStepSettingsProps {
   toolId: string;