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.

packages/ai/src/background-removal.ts

@@ -0,0 +1,30 @@
+import { runPythonScript } from "./bridge.js";
+import { writeFile, readFile } from "node:fs/promises";
+import { join } from "node:path";
+
+export interface RemoveBackgroundOptions {
+  model?: "u2net" | "isnet";
+}
+
+export async function removeBackground(
+  inputBuffer: Buffer,
+  outputDir: string,
+  options: RemoveBackgroundOptions = {},
+): Promise<Buffer> {
+  const inputPath = join(outputDir, "input_bg.png");
+  const outputPath = join(outputDir, "output_bg.png");
+
+  await writeFile(inputPath, inputBuffer);
+  const { stdout } = await runPythonScript("remove_bg.py", [
+    inputPath,
+    outputPath,
+    JSON.stringify(options),
+  ]);
+
+  const result = JSON.parse(stdout);
+  if (!result.success) {
+    throw new Error(result.error || "Background removal failed");
+  }
+
+  return readFile(outputPath);
+}