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

tests/e2e/tools-all.spec.ts

@@ -16,10 +16,7 @@ const TOOLS_WITH_DROPZONE = [
   { id: "bulk-rename", name: "Bulk Rename" },
   { id: "image-to-pdf", name: "Image to PDF" },
   { id: "favicon", name: "Favicon" },
-  { id: "brightness-contrast", name: "Brightness" },
-  { id: "saturation", name: "Saturation" },
-  { id: "color-channels", name: "Color Channels" },
-  { id: "color-effects", name: "Color Effects" },
+  { id: "adjust-colors", name: "Adjust Colors" },
   { id: "replace-color", name: "Replace" },
   { id: "remove-background", name: "Remove Background" },
   { id: "upscale", name: "Upscal" },
@@ -88,7 +85,7 @@ test.describe("Tool pages accept file upload", () => {
     "compress",
     "convert",
     "strip-metadata",
-    "brightness-contrast",
+    "adjust-colors",
     "watermark-text",
     "info",
     "border",