feat: add external API for publishing posts with image URL and scheduling

- New POST /api/v1/publish endpoint: download image from URL, create post and schedule it per page
- New GET /api/v1/pages endpoint: list available pages for external callers
- API key auth via X-API-Key header, configurable from admin settings (stored in DB)
- New app_config table (migration included) to store the external API key
- Admin-only settings section (desktop + mobile) to set/revoke the key
- Fallback to EXTERNAL_API_KEY env var if no DB key is configured

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

server/migrate.ts

@@ -142,6 +142,16 @@ export async function migrate() {
       );
     `);
 
+    // app_config (global application settings)
+    await client.query(`
+      CREATE TABLE IF NOT EXISTS "app_config" (
+        "id" varchar PRIMARY KEY DEFAULT gen_random_uuid() NOT NULL,
+        "external_api_key" text,
+        "created_at" timestamp DEFAULT now(),
+        "updated_at" timestamp DEFAULT now()
+      );
+    `);
+
     console.log("[Migration] Safe migration completed.");
   } catch (error) {
     console.error("[Migration] Error during migration:", error);