feat: Add Chat UX improvements with notifications and @mention support

- Add ActionBar component with expandable toolbar for mobile
- Add @mention functionality with autocomplete dropdown
- Add browser notification system (push, sound, vibration)
- Add NotificationSettings modal for user preferences
- Add mention badges on room list cards
- Add ReportPreview with Markdown rendering and copy/download
- Add message copy functionality with hover actions
- Add backend mentions field to messages with Alembic migration
- Add lots field to rooms, remove templates
- Optimize WebSocket database session handling
- Various UX polish (animations, accessibility)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/core/config.py

@@ -61,6 +61,12 @@ class Settings(BaseSettings):
     REPORT_MAX_MESSAGES: int = 200  # Summarize if exceeded
     REPORT_STORAGE_PATH: str = "reports"  # MinIO path prefix for reports
 
+    # Database Connection Pool
+    DB_POOL_SIZE: int = 20  # Number of persistent connections
+    DB_MAX_OVERFLOW: int = 30  # Max additional connections beyond pool_size
+    DB_POOL_TIMEOUT: int = 10  # Seconds to wait for available connection
+    DB_POOL_RECYCLE: int = 1800  # Recycle connections after 30 minutes
+
     @field_validator("LOG_LEVEL")
     @classmethod
     def validate_log_level(cls, v: str) -> str: