feat: add server-sent events for logging and activity tracking

- Implemented a custom tracing layer (`AppLogLayer`) to capture log events and broadcast them to SSE clients.
- Created admin routes for streaming server logs and listing recent activity logs.
- Added an activity log repository interface and SQLite implementation for persisting activity events.
- Integrated activity logging into user authentication and channel CRUD operations.
- Developed frontend components for displaying server logs and activity logs in the admin panel.
- Enhanced the video player with a stats overlay for monitoring streaming metrics.

k-tv-backend/migrations_sqlite/20260317000000_add_activity_log.sql

@@ -0,0 +1,9 @@
+CREATE TABLE IF NOT EXISTS activity_log (
+    id TEXT PRIMARY KEY NOT NULL,
+    timestamp TEXT NOT NULL,
+    event_type TEXT NOT NULL,
+    detail TEXT NOT NULL,
+    channel_id TEXT
+);
+
+CREATE INDEX IF NOT EXISTS idx_activity_log_timestamp ON activity_log(timestamp DESC);