fix(api): remove broken legacy /system/mqtt/settings route

The GET/PUT /api/v1/system/mqtt/settings handlers read cfg.mqtt.*, but the
single-broker MQTTConfig block was removed in the multi-broker refactor, so any
call raised AttributeError. Brokers are now first-class MQTTSource entities
managed via the mqtt.py router, and the frontend no longer calls this endpoint.
Remove the dead handlers, the _load_mqtt_settings helper, the now-unused
get_config import, and the orphaned MQTTSettings{Request,Response} schemas.

server/src/ledgrab/api/routes/system_settings.py

@@ -1,4 +1,4 @@
-"""System routes: MQTT, external URL, ADB, logs WebSocket, log level.
+"""System routes: external URL, shutdown action, ADB, logs WebSocket, log level.
 
 Extracted from system.py to keep files under 800 lines.
 """
@@ -17,13 +17,10 @@ from ledgrab.api.schemas.system import (
     ExternalUrlResponse,
     LogLevelRequest,
     LogLevelResponse,
-    MQTTSettingsRequest,
-    MQTTSettingsResponse,
     ShutdownAction,
     ShutdownActionRequest,
     ShutdownActionResponse,
 )
-from ledgrab.config import get_config
 from ledgrab.storage.database import Database
 from ledgrab.utils import get_logger
 
@@ -32,85 +29,6 @@ logger = get_logger(__name__)
 router = APIRouter()
 
 
-# ---------------------------------------------------------------------------
-# MQTT settings
-# ---------------------------------------------------------------------------
-
-
-def _load_mqtt_settings(db: Database) -> dict:
-    """Load MQTT settings: YAML config defaults overridden by DB settings."""
-    cfg = get_config()
-    defaults = {
-        "enabled": cfg.mqtt.enabled,
-        "broker_host": cfg.mqtt.broker_host,
-        "broker_port": cfg.mqtt.broker_port,
-        "username": cfg.mqtt.username,
-        "password": cfg.mqtt.password,
-        "client_id": cfg.mqtt.client_id,
-        "base_topic": cfg.mqtt.base_topic,
-    }
-    overrides = db.get_setting("mqtt")
-    if overrides:
-        defaults.update(overrides)
-    return defaults
-
-
-@router.get(
-    "/api/v1/system/mqtt/settings",
-    response_model=MQTTSettingsResponse,
-    tags=["System"],
-)
-async def get_mqtt_settings(_: AuthRequired, db: Database = Depends(get_database)):
-    """Get current MQTT broker settings. Password is masked."""
-    s = _load_mqtt_settings(db)
-    return MQTTSettingsResponse(
-        enabled=s["enabled"],
-        broker_host=s["broker_host"],
-        broker_port=s["broker_port"],
-        username=s["username"],
-        password_set=bool(s.get("password")),
-        client_id=s["client_id"],
-        base_topic=s["base_topic"],
-    )
-
-
-@router.put(
-    "/api/v1/system/mqtt/settings",
-    response_model=MQTTSettingsResponse,
-    tags=["System"],
-)
-async def update_mqtt_settings(
-    _: AuthRequired, body: MQTTSettingsRequest, db: Database = Depends(get_database)
-):
-    """Update MQTT broker settings. If password is empty string, the existing password is preserved."""
-    current = _load_mqtt_settings(db)
-
-    # If caller sends an empty password, keep the existing one
-    password = body.password if body.password else current.get("password", "")
-
-    new_settings = {
-        "enabled": body.enabled,
-        "broker_host": body.broker_host,
-        "broker_port": body.broker_port,
-        "username": body.username,
-        "password": password,
-        "client_id": body.client_id,
-        "base_topic": body.base_topic,
-    }
-    db.set_setting("mqtt", new_settings)
-    logger.info("MQTT settings updated")
-
-    return MQTTSettingsResponse(
-        enabled=new_settings["enabled"],
-        broker_host=new_settings["broker_host"],
-        broker_port=new_settings["broker_port"],
-        username=new_settings["username"],
-        password_set=bool(new_settings["password"]),
-        client_id=new_settings["client_id"],
-        base_topic=new_settings["base_topic"],
-    )
-
-
 # ---------------------------------------------------------------------------
 # External URL setting
 # ---------------------------------------------------------------------------

server/src/ledgrab/api/schemas/system.py

@@ -194,35 +194,6 @@ class BackupListResponse(BaseModel):
     count: int
 
 
-# ─── MQTT schemas ──────────────────────────────────────────────
-
-
-class MQTTSettingsResponse(BaseModel):
-    """MQTT broker settings response (password is masked)."""
-
-    enabled: bool = Field(description="Whether MQTT is enabled")
-    broker_host: str = Field(description="MQTT broker hostname or IP")
-    broker_port: int = Field(ge=1, le=65535, description="MQTT broker port")
-    username: str = Field(description="MQTT username (empty = anonymous)")
-    password_set: bool = Field(description="Whether a password is configured")
-    client_id: str = Field(description="MQTT client ID")
-    base_topic: str = Field(description="Base topic prefix")
-
-
-class MQTTSettingsRequest(BaseModel):
-    """MQTT broker settings update request."""
-
-    enabled: bool = Field(description="Whether MQTT is enabled")
-    broker_host: str = Field(description="MQTT broker hostname or IP")
-    broker_port: int = Field(ge=1, le=65535, description="MQTT broker port")
-    username: str = Field(default="", description="MQTT username (empty = anonymous)")
-    password: str = Field(
-        default="", description="MQTT password (empty = keep existing if omitted)"
-    )
-    client_id: str = Field(default="ledgrab", description="MQTT client ID")
-    base_topic: str = Field(default="ledgrab", description="Base topic prefix")
-
-
 # ─── External URL schema ───────────────────────────────────────