feat: add per-device custom Chrome production URL override

- Device model: add custom_chrome_url nullable column
- API: GET /api/wmt/config/<mac> returns device custom_chrome_url when set,
  falls back to WMTGlobalConfig.chrome_url for all other devices
- Web: device_edit POST saves/clears custom_chrome_url from form
- Template: wmt/device_form.html adds Chrome URL override section with
  clear status indicator (using global vs custom URL)
- DB: ALTER TABLE devices ADD COLUMN custom_chrome_url VARCHAR(500) applied

app/models/__init__.py

@@ -52,6 +52,7 @@ class Device(Base):
     config_synced_at = Column(DateTime)        # set by server when client confirms in-sync
     info_reviewed_at = Column(DateTime, default=lambda: datetime(1970, 1, 1))
     card_presence = Column(String(10), default='enable')
+    custom_chrome_url = Column(String(500), nullable=True)  # per-device production URL override (overrides WMTGlobalConfig.chrome_url)
 
     # Relationships
     logs = relationship("LogEntry", back_populates="device")