feat(voice): per-user TTS voice selection via /목소리

Until now every TTS message was synthesized with VoiceType.가람
regardless of who spoke. This adds a user-scoped voice preference
persisted in SQLite so each member can pick their own voice and
keep it across bot restarts.

Changes
- db/schema.sql: add nullable `voice_type` column to `users`.
- src/utils/Database.ts:
  - run a PRAGMA-driven ALTER TABLE migration so existing DBs gain
    the column without dropping data.
  - add `DB.user.setVoice(guildId, userId, name, voiceType)` that
    upserts the row.
- src/classes/TTSClient.ts:
  - export `DEFAULT_VOICE` (= 가람).
  - resolve the speaking member's stored voice in `tts()` and
    thread it through `getSource()` instead of hardcoding 가람.
  - validate stored slug against `VoiceType` so stale/unknown
    values silently fall back to the default.
- src/commands/voice.ts (new):
  - `/목소리` slash command shows the user's current voice and a
    StringSelectMenu of all `VoiceType` entries (현재 + 이전 보이스
    모두). Selection writes to `users.voice_type` and confirms
    ephemerally.
  - defensively creates a guild row if `/tts channel register`
    hasn't run yet, to satisfy the ON DELETE CASCADE FK.

Deploy
Run `npm run prod` after pulling so Discord sees the new
`/목소리` command. No env or config changes required.

src/utils/Database.ts

@@ -13,6 +13,15 @@ const schemaPath = join(process.cwd(), "db/schema.sql");
 const schema = readFileSync(schemaPath, "utf-8");
 database.exec(schema);
 
+// 마이그레이션: 기존 DB 에 voice_type 컬럼이 없으면 추가
+// (CREATE TABLE IF NOT EXISTS 는 이미 존재하는 테이블에 컬럼을 추가하지 않으므로
+//  PRAGMA 로 직접 확인한다.)
+const userCols = database.prepare("PRAGMA table_info(users)").all() as { name: string }[];
+if (!userCols.some(c => c.name === "voice_type")) {
+  database.exec("ALTER TABLE users ADD COLUMN voice_type TEXT");
+  Logger.ready("DB 마이그레이션: users.voice_type 컬럼 추가");
+}
+
 Logger.ready("DB 활성화!");
 
 const stmt = {
@@ -108,5 +117,21 @@ export const DB = {
         return false;
       }
     },
+    // 유저 행이 없으면 insert, 있으면 update.
+    // (sqlite UPSERT 대신 명시적 분기 — 기존 set/update 패턴 유지)
+    setVoice(guildId: string, id: string, name: string, voiceType: string) {
+      try {
+        const existing = stmt.user.get.get(guildId, id) as UserType | undefined;
+        if (existing) {
+          stmt.user.update({ guild_id: guildId, id, name, voice_type: voiceType });
+        } else {
+          stmt.user.insert({ guild_id: guildId, id, name, voice_type: voiceType });
+        }
+        return true;
+      } catch (err) {
+        Logger.error(String(err));
+        return false;
+      }
+    },
   },
 };