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/api/src/routes/tools/erase-object.ts

@@ -0,0 +1,88 @@
+import type { FastifyInstance, FastifyRequest, FastifyReply } from "fastify";
+import { randomUUID } from "node:crypto";
+import { writeFile } from "node:fs/promises";
+import { join, basename } from "node:path";
+import { inpaint } from "@stirling-image/ai";
+import { createWorkspace } from "../../lib/workspace.js";
+
+/**
+ * Object eraser / inpainting route.
+ * Accepts an image and a mask image, erases masked areas.
+ */
+export function registerEraseObject(app: FastifyInstance) {
+  app.post(
+    "/api/v1/tools/erase-object",
+    async (request: FastifyRequest, reply: FastifyReply) => {
+      let imageBuffer: Buffer | null = null;
+      let maskBuffer: Buffer | null = null;
+      let filename = "image";
+
+      try {
+        const parts = request.parts();
+        for await (const part of parts) {
+          if (part.type === "file") {
+            const chunks: Buffer[] = [];
+            for await (const chunk of part.file) {
+              chunks.push(chunk);
+            }
+            const buf = Buffer.concat(chunks);
+            if (part.fieldname === "mask") {
+              maskBuffer = buf;
+            } else {
+              imageBuffer = buf;
+              filename = basename(part.filename ?? "image");
+            }
+          }
+        }
+      } catch (err) {
+        return reply.status(400).send({
+          error: "Failed to parse multipart request",
+          details: err instanceof Error ? err.message : String(err),
+        });
+      }
+
+      if (!imageBuffer || imageBuffer.length === 0) {
+        return reply.status(400).send({ error: "No image file provided" });
+      }
+      if (!maskBuffer || maskBuffer.length === 0) {
+        return reply
+          .status(400)
+          .send({ error: "No mask image provided. Upload a mask as a second file with fieldname 'mask'" });
+      }
+
+      try {
+        const jobId = randomUUID();
+        const workspacePath = await createWorkspace(jobId);
+
+        // Save input
+        const inputPath = join(workspacePath, "input", filename);
+        await writeFile(inputPath, imageBuffer);
+
+        // Process
+        const resultBuffer = await inpaint(
+          imageBuffer,
+          maskBuffer,
+          join(workspacePath, "output"),
+        );
+
+        // Save output
+        const outputFilename =
+          filename.replace(/\.[^.]+$/, "") + "_erased.png";
+        const outputPath = join(workspacePath, "output", outputFilename);
+        await writeFile(outputPath, resultBuffer);
+
+        return reply.send({
+          jobId,
+          downloadUrl: `/api/v1/download/${jobId}/${encodeURIComponent(outputFilename)}`,
+          originalSize: imageBuffer.length,
+          processedSize: resultBuffer.length,
+        });
+      } catch (err) {
+        return reply.status(422).send({
+          error: "Object erasing failed",
+          details: err instanceof Error ? err.message : "Unknown error",
+        });
+      }
+    },
+  );
+}