Add alert rules with FilterBar-based triggers and ntfy delivery (tickets 017-019)

Adds AlertRule/AlertCondition/AlertSink models with CRUD persisted on
Settings (mirroring CssEntry's shape), per-line trigger evaluation hooked
into the engine's ingest path (Immediate/Threshold/NewValue/Ratio
conditions), and best-effort ntfy delivery spawned off the ingest hot path.

Design notes:
- Alert queries reuse FilterBar syntax but only support the
  regex()/sequence/timestamp subset (no metric[param] comparisons), since
  resolving those would require a full bucket rescan per alert per ingested
  line rather than cost proportional to new logs.
- Ratio's zero-denominator case skips evaluation for that line rather than
  treating the ratio as infinite.
- NewValue's seen-value set is in-memory only and does not survive a
  restart, matching the engine's other non-persisted runtime state.

engine::run now takes a SettingsHandle (moved from api.rs to settings.rs so
engine.rs can depend on it without a circular import) to read the current
alert rules on each ingest.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

src/profile.rs

@@ -932,7 +932,8 @@ mod tests {
         let default_profile = store.get(DEFAULT_PROFILE_NAME).unwrap().clone();
 
         let (engine_tx, engine_rx) = mpsc::channel(128);
-        tokio::spawn(engine::run(engine_rx));
+        let settings_handle = std::sync::Arc::new(std::sync::Mutex::new(settings.clone()));
+        tokio::spawn(engine::run(engine_rx, settings_handle));
         load_into_engine(&engine_tx, &default_profile).await;
 
         for (sequence, text) in [(0u64, "all good"), (1, "an ERROR happened"), (2, "fine again")] {
@@ -1130,7 +1131,8 @@ mod tests {
 
         // Spawn a fresh engine and replay the profile's declarations into it.
         let (engine_tx, engine_rx) = mpsc::channel(16);
-        tokio::spawn(crate::engine::run(engine_rx));
+        let settings_handle = std::sync::Arc::new(std::sync::Mutex::new(settings.clone()));
+        tokio::spawn(crate::engine::run(engine_rx, settings_handle));
         load_into_engine(&engine_tx, &profile).await;
 
         // Ingest a line and confirm the engine is functioning with the replayed