refactor: split api.py monolith into 19 route modules (Phase B)

Split 3,598-line api.py into thin orchestrator (128 lines) + 19 route
modules in bot/routes/:

  core.py (7 routes), mood.py (10), language.py (3), evil_mode.py (6),
  bipolar_mode.py (9), gpu.py (2), bot_actions.py (4), autonomous.py (13),
  profile_picture.py (26), manual_send.py (3), servers.py (6),
  figurines.py (5), dms.py (18), image_generation.py (4), chat.py (1),
  config.py (7), logging_config.py (9), voice.py (3), memory.py (10)

All 146 routes verified present via test_route_split.py (149 tests).
21/21 regression tests (test_config_state.py) pass.
Monolith backup: bot/api_monolith_backup.py (revert: cp it to api.py).

bot/routes/language.py

@@ -0,0 +1,74 @@
+"""Language mode routes."""
+
+from fastapi import APIRouter
+import globals
+from utils.logger import get_logger
+
+logger = get_logger('api')
+
+router = APIRouter()
+
+
+@router.get("/language")
+def get_language_mode():
+    """Get current language mode (english or japanese)"""
+    return {
+        "language_mode": globals.LANGUAGE_MODE,
+        "available_languages": ["english", "japanese"],
+        "current_model": globals.JAPANESE_TEXT_MODEL if globals.LANGUAGE_MODE == "japanese" else globals.TEXT_MODEL
+    }
+
+
+@router.post("/language/toggle")
+def toggle_language_mode():
+    """Toggle between English and Japanese modes"""
+    if globals.LANGUAGE_MODE == "english":
+        globals.LANGUAGE_MODE = "japanese"
+        new_mode = "japanese"
+        model_used = globals.JAPANESE_TEXT_MODEL
+        logger.info("Switched to Japanese mode (using Llama 3.1 Swallow)")
+    else:
+        globals.LANGUAGE_MODE = "english"
+        new_mode = "english"
+        model_used = globals.TEXT_MODEL
+        logger.info("Switched to English mode (using default model)")
+
+    # Persist via config manager
+    try:
+        from config_manager import config_manager
+        config_manager.set("discord.language_mode", new_mode, persist=True)
+        logger.info(f"💾 Language mode persisted to config_runtime.yaml")
+    except Exception as e:
+        logger.warning(f"Failed to persist language mode: {e}")
+
+    return {
+        "status": "ok",
+        "language_mode": new_mode,
+        "model_now_using": model_used,
+        "message": f"Miku is now speaking in {new_mode.upper()}!"
+    }
+
+
+@router.post("/language/set")
+def set_language_mode(language: str = "english"):
+    """Set language mode to either 'english' or 'japanese'"""
+    if language.lower() not in ["english", "japanese"]:
+        return {"error": f"Invalid language mode '{language}'. Use 'english' or 'japanese'."}, 400
+
+    globals.LANGUAGE_MODE = language.lower()
+    model_used = globals.JAPANESE_TEXT_MODEL if language.lower() == "japanese" else globals.TEXT_MODEL
+    logger.info(f"Language mode set to {language.lower()} (using {model_used})")
+
+    # Persist so it survives restarts
+    try:
+        from config_manager import config_manager
+        config_manager.set("discord.language_mode", language.lower(), persist=True)
+    except Exception:
+        pass
+
+    return {
+        "status": "ok",
+        "language_mode": language.lower(),
+        "model_now_using": model_used,
+        "message": f"Miku is now speaking in {language.upper()}!"
+    }