feat: Gemini image generation in Workshop chat (#19)

- Add image intent detection (draw/illustrate/visualize/create an image)
  via `detectImageRequest()` in agent.ts; exports used by jobs and sessions
- Add `executeImageWork()` to AgentService: calls Gemini generateImage with
  graceful fallback stub PNG when Gemini credentials are absent
- Add `job_media` table (migration 0010) for base64 image storage with 7-day TTL;
  entity_id is polymorphic for both jobs and session requests
- Add `media_type TEXT` column to jobs table (flagged during eval phase)
- Add `calculateImageFeeSats()` / `calculateImageFeeUsd()` to PricingService;
  uses IMAGE_GENERATION_FLAT_RATE_USD env var (default $0.04)
- Jobs route: detect image jobs in eval phase, route to Gemini in execution,
  store image in job_media; expose GET /api/jobs/:id/media endpoint
- Sessions route: detect image requests, call executeImageWork, store in
  job_media, return mediaUrl and mediaType in response
- Estimate route: return image pricing and mediaType:'image' for image requests
- Event bus: add optional mediaUrl/mediaType to job:completed event
- Frontend session.js: render generated images inline with download button

Fixes #19

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

artifacts/api-server/src/lib/agent.ts

@@ -2,6 +2,23 @@ import { makeLogger } from "./logger.js";
 
 const logger = makeLogger("agent");
 
+// ── Image request detection ───────────────────────────────────────────────────
+
+const IMAGE_INTENT_RE =
+  /\b(draw|illustrate|create\s+an?\s+image\s+of|generate\s+an?\s+image\s+of|visualize|visualise|make\s+an?\s+image\s+of|paint\s+me|sketch|render\s+an?\s+image\s+of|picture\s+of)\b/i;
+
+/**
+ * Returns true if the request text signals an image-generation intent.
+ */
+export function detectImageRequest(text: string): boolean {
+  return IMAGE_INTENT_RE.test(text);
+}
+
+export interface ImageWorkResult {
+  b64_json: string;
+  mimeType: string;
+}
+
 export interface EvalResult {
   accepted: boolean;
   reason: string;
@@ -442,6 +459,36 @@ Respond ONLY with valid JSON: {"accepted": true/false, "reason": "..."}`,
       return "";
     }
   }
+
+  /**
+   * Generate an image via Gemini for the given prompt.
+   * Falls back to a stub 1×1 transparent PNG when Gemini credentials are absent.
+   */
+  async executeImageWork(prompt: string): Promise<ImageWorkResult> {
+    const geminiAvailable =
+      !!process.env["AI_INTEGRATIONS_GEMINI_API_KEY"] &&
+      !!process.env["AI_INTEGRATIONS_GEMINI_BASE_URL"];
+
+    if (!geminiAvailable) {
+      logger.warn("Gemini credentials absent — returning stub image", { component: "agent" });
+      // 1×1 transparent PNG (base64)
+      return {
+        b64_json:
+          "iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNk+M9QDwADhgGAWjR9awAAAABJRU5ErkJggg==",
+        mimeType: "image/png",
+      };
+    }
+
+    try {
+      const mod = (await import("@workspace/integrations-gemini-ai")) as {
+        generateImage: (prompt: string) => Promise<{ b64_json: string; mimeType: string }>;
+      };
+      return await mod.generateImage(prompt);
+    } catch (err) {
+      logger.error("Gemini image generation failed", { error: String(err) });
+      throw err;
+    }
+  }
 }
 
 export const agentService = new AgentService();