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/api/src/log_layer.rs

@@ -0,0 +1,72 @@
+//! Custom tracing layer that captures log events and broadcasts them to SSE clients.
+
+use chrono::Utc;
+use serde::Serialize;
+use std::collections::VecDeque;
+use std::sync::{Arc, Mutex};
+use tokio::sync::broadcast;
+use tracing::Event;
+use tracing_subscriber::Layer;
+
+/// A single structured log line sent to SSE clients.
+#[derive(Debug, Clone, Serialize)]
+pub struct LogLine {
+    pub level: String,
+    pub target: String,
+    pub message: String,
+    pub timestamp: String,
+}
+
+/// Tracing layer that fans log events out to a broadcast channel + ring buffer.
+pub struct AppLogLayer {
+    tx: broadcast::Sender<LogLine>,
+    history: Arc<Mutex<VecDeque<LogLine>>>,
+}
+
+impl AppLogLayer {
+    pub fn new(
+        tx: broadcast::Sender<LogLine>,
+        history: Arc<Mutex<VecDeque<LogLine>>>,
+    ) -> Self {
+        Self { tx, history }
+    }
+}
+
+impl<S: tracing::Subscriber> Layer<S> for AppLogLayer {
+    fn on_event(&self, event: &Event<'_>, _ctx: tracing_subscriber::layer::Context<'_, S>) {
+        let mut visitor = MsgVisitor(String::new());
+        event.record(&mut visitor);
+
+        let line = LogLine {
+            level: event.metadata().level().to_string(),
+            target: event.metadata().target().to_string(),
+            message: visitor.0,
+            timestamp: Utc::now().to_rfc3339(),
+        };
+
+        if let Ok(mut history) = self.history.lock() {
+            if history.len() >= 200 {
+                history.pop_front();
+            }
+            history.push_back(line.clone());
+        }
+
+        let _ = self.tx.send(line);
+    }
+}
+
+struct MsgVisitor(String);
+
+impl tracing::field::Visit for MsgVisitor {
+    fn record_str(&mut self, field: &tracing::field::Field, value: &str) {
+        if field.name() == "message" {
+            self.0 = value.to_owned();
+        }
+    }
+
+    fn record_debug(&mut self, field: &tracing::field::Field, value: &dyn std::fmt::Debug) {
+        if field.name() == "message" {
+            self.0 = format!("{value:?}");
+        }
+    }
+}