feat: add Phase 4 AI tools with Python bridge and 6 new tools

Add Python bridge (packages/ai/src/bridge.ts) that calls Python scripts
via child_process with venv-first fallback to system python3. Implements
6 AI-powered tools:

- Remove Background: rembg-based with U2-Net/IS-Net models
- Image Upscaling: Real-ESRGAN with Lanczos fallback
- OCR/Text Extraction: Tesseract + PaddleOCR engines
- Face/PII Blur: MediaPipe face detection with configurable blur
- Object Eraser: LaMa inpainting with mask-based input
- Smart Crop: Sharp attention-based entropy cropping (no Python needed)

Each tool includes: Python script, TypeScript wrapper, API route,
and React settings component. All Python scripts handle ImportError
gracefully with clear installation messages.

apps/web/src/components/tools/blur-faces-settings.tsx

@@ -0,0 +1,104 @@
+import { useState } from "react";
+import { useFileStore } from "@/stores/file-store";
+import { useToolProcessor } from "@/hooks/use-tool-processor";
+import { Download, Loader2 } from "lucide-react";
+
+export function BlurFacesSettings() {
+  const { files } = useFileStore();
+  const { processFiles, processing, error, downloadUrl, originalSize, processedSize } =
+    useToolProcessor("blur-faces");
+
+  const [blurRadius, setBlurRadius] = useState(30);
+  const [sensitivity, setSensitivity] = useState(50);
+
+  const handleProcess = () => {
+    processFiles(files, {
+      blurRadius,
+      sensitivity: sensitivity / 100,
+    });
+  };
+
+  const hasFile = files.length > 0;
+
+  return (
+    <div className="space-y-4">
+      {/* Blur radius */}
+      <div>
+        <div className="flex justify-between items-center">
+          <label className="text-xs text-muted-foreground">Blur Radius</label>
+          <span className="text-xs font-mono text-foreground">{blurRadius}</span>
+        </div>
+        <input
+          type="range"
+          min={5}
+          max={80}
+          value={blurRadius}
+          onChange={(e) => setBlurRadius(Number(e.target.value))}
+          className="w-full mt-1"
+        />
+        <div className="flex justify-between text-[10px] text-muted-foreground mt-0.5">
+          <span>Light</span>
+          <span>Heavy</span>
+        </div>
+      </div>
+
+      {/* Sensitivity */}
+      <div>
+        <div className="flex justify-between items-center">
+          <label className="text-xs text-muted-foreground">Detection Sensitivity</label>
+          <span className="text-xs font-mono text-foreground">{sensitivity}%</span>
+        </div>
+        <input
+          type="range"
+          min={10}
+          max={90}
+          value={sensitivity}
+          onChange={(e) => setSensitivity(Number(e.target.value))}
+          className="w-full mt-1"
+        />
+        <div className="flex justify-between text-[10px] text-muted-foreground mt-0.5">
+          <span>More faces</span>
+          <span>Fewer false positives</span>
+        </div>
+      </div>
+
+      {/* Info */}
+      <p className="text-[10px] text-muted-foreground">
+        Uses MediaPipe for face detection. Automatically detects and blurs all faces in the image.
+      </p>
+
+      {/* Error */}
+      {error && <p className="text-xs text-red-500">{error}</p>}
+
+      {/* Size info */}
+      {originalSize != null && processedSize != null && (
+        <div className="text-xs text-muted-foreground space-y-0.5">
+          <p>Original: {(originalSize / 1024).toFixed(1)} KB</p>
+          <p>Processed: {(processedSize / 1024).toFixed(1)} KB</p>
+        </div>
+      )}
+
+      {/* Process button */}
+      <button
+        onClick={handleProcess}
+        disabled={!hasFile || processing}
+        className="w-full py-2.5 rounded-lg bg-primary text-primary-foreground font-medium disabled:opacity-50 disabled:cursor-not-allowed flex items-center justify-center gap-2"
+      >
+        {processing && <Loader2 className="h-4 w-4 animate-spin" />}
+        {processing ? "Detecting Faces..." : "Blur Faces"}
+      </button>
+
+      {/* Download */}
+      {downloadUrl && (
+        <a
+          href={downloadUrl}
+          download
+          className="w-full py-2.5 rounded-lg border border-primary text-primary font-medium flex items-center justify-center gap-2 hover:bg-primary/5"
+        >
+          <Download className="h-4 w-4" />
+          Download
+        </a>
+      )}
+    </div>
+  );
+}