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/bridge.ts

@@ -0,0 +1,86 @@
+import { execFile } from "node:child_process";
+import { promisify } from "node:util";
+import { resolve, dirname } from "node:path";
+import { fileURLToPath } from "node:url";
+
+const execFileAsync = promisify(execFile);
+
+const __dirname = dirname(fileURLToPath(import.meta.url));
+const PYTHON_DIR = resolve(__dirname, "../python");
+
+/** Try venv first, then system python. */
+function getPythonPath(): string {
+  const venvPath = process.env.PYTHON_VENV_PATH || "/opt/venv";
+  return `${venvPath}/bin/python3`;
+}
+
+/**
+ * Extract a user-friendly error from a Python process error.
+ * Python scripts print JSON to stderr/stdout on failure — try to parse it.
+ */
+function extractPythonError(error: unknown): string {
+  if (error && typeof error === "object") {
+    const execError = error as {
+      stderr?: string;
+      stdout?: string;
+      message?: string;
+    };
+    // Try to parse JSON error from stderr or stdout
+    for (const output of [execError.stderr, execError.stdout]) {
+      if (output) {
+        try {
+          const parsed = JSON.parse(output.trim());
+          if (parsed.error) return parsed.error;
+        } catch {
+          // Not JSON, check for human-readable content
+          const trimmed = output.trim();
+          if (trimmed && !trimmed.startsWith("Traceback")) {
+            return trimmed;
+          }
+        }
+      }
+    }
+    if (execError.message) return execError.message;
+  }
+  return String(error);
+}
+
+/**
+ * Run a Python script from packages/ai/python/ with the given arguments.
+ * Falls back to system python3 if the venv is not available.
+ */
+export async function runPythonScript(
+  scriptName: string,
+  args: string[],
+  timeoutMs = 300000, // 5 min default
+): Promise<{ stdout: string; stderr: string }> {
+  const scriptPath = resolve(PYTHON_DIR, scriptName);
+  const pythonPath = getPythonPath();
+
+  const execOpts = {
+    timeout: timeoutMs,
+    maxBuffer: 50 * 1024 * 1024, // 50MB
+  };
+
+  try {
+    const { stdout, stderr } = await execFileAsync(
+      pythonPath,
+      [scriptPath, ...args],
+      execOpts,
+    );
+    return { stdout: stdout.trim(), stderr: stderr.trim() };
+  } catch {
+    // Try system python as fallback
+    try {
+      const { stdout, stderr } = await execFileAsync(
+        "python3",
+        [scriptPath, ...args],
+        execOpts,
+      );
+      return { stdout: stdout.trim(), stderr: stderr.trim() };
+    } catch (fallbackError: unknown) {
+      const message = extractPythonError(fallbackError);
+      throw new Error(message);
+    }
+  }
+}