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/App.tsx

@@ -106,6 +106,14 @@ export function App() {
               <Route path="/files" element={<FilesPage />} />
               <Route path="/fullscreen" element={<FullscreenGridPage />} />
               <Route path="/privacy" element={<PrivacyPolicyPage />} />
+              {/* Redirects: old color tools consolidated into adjust-colors */}
+              <Route
+                path="/brightness-contrast"
+                element={<Navigate to="/adjust-colors" replace />}
+              />
+              <Route path="/saturation" element={<Navigate to="/adjust-colors" replace />} />
+              <Route path="/color-channels" element={<Navigate to="/adjust-colors" replace />} />
+              <Route path="/color-effects" element={<Navigate to="/adjust-colors" replace />} />
               <Route path="/:toolId" element={<ToolPage />} />
               <Route path="/" element={<HomePage />} />
             </Routes>