refactor(tts): replace Minimax+Freesound with Piper TTS

Remove Minimax Speech API and Freesound integrations entirely.
Add Piper TTS as the sole TTS provider via configurable HTTP URL.

- Add piper_config DB table (url field, per-user)
- Add GET/POST /api/piper/config routes
- Python: replace generate_tts_minimax with generate_tts_piper (GET ?text=, WAV→MP3)
- Simplify generate_tts_with_subs: piper_url param replaces tts_provider+minimax fields
- Drop ttsVoice/ttsProvider from all UI, API, and background job params
- Settings page: replace Minimax+Freesound cards with single Piper URL input
- Remove freeSoundService init from server startup and CSP headers

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

server/migrate.ts

@@ -152,24 +152,18 @@ export async function migrate() {
       );
     `);
 
-    // minimax_config
+    // piper_config
     await client.query(`
-      CREATE TABLE IF NOT EXISTS "minimax_config" (
+      CREATE TABLE IF NOT EXISTS "piper_config" (
         "id" varchar PRIMARY KEY DEFAULT gen_random_uuid() NOT NULL,
         "user_id" varchar NOT NULL REFERENCES "users"("id") ON DELETE cascade,
-        "api_key" text NOT NULL,
+        "url" text NOT NULL,
         "created_at" timestamp DEFAULT now(),
         "updated_at" timestamp DEFAULT now(),
-        CONSTRAINT minimax_config_user_id_unique UNIQUE ("user_id")
+        CONSTRAINT piper_config_user_id_unique UNIQUE ("user_id")
       );
     `);
 
-    // minimax_config.group_id
-    await client.query(`
-      ALTER TABLE "minimax_config"
-      ADD COLUMN IF NOT EXISTS "group_id" text;
-    `);
-
     console.log("[Migration] Safe migration completed.");
   } catch (error) {
     console.error("[Migration] Error during migration:", error);