fix(monitor): serialize DB writes through a single drained writer

Every check spawned `go e.db.Save*(...)` with the error discarded: a
fire-and-forget goroutine per log line, check, state change, and alert
health update. SaveLog ran a full-table prune DELETE on every insert and
SaveCheck a COUNT + conditional prune on every check, so the hot path
amplified each write into several statements. Nothing tracked these
goroutines, so at shutdown they raced the store's Close() — writes to a
closing DB, silently swallowed.

Introduce a single writer goroutine that drains a buffered channel of
typed dbWrite values (log/check/state-change/alert-health). Writes are
enqueued non-blocking; a saturated queue drops and notes it in the
in-memory log rather than blocking the check loop. Write errors are now
logged instead of discarded. Retention moves off the hot path: SaveLog
and SaveCheck become plain INSERTs, and PruneLogs/PruneCheckHistory/
PruneStateChanges run on a 10-minute timer inside the writer (single
keep-newest-N-per-site pass via a window function). state_changes was
previously never pruned — now bounded.

Add Engine.Stop(): cancels the engine's context, then waits for the
writer to drain every buffered write before returning. main wires it in
before the deferred store Close() so no write races a closed DB.

SQLite gains busy_timeout=5000 and synchronous=NORMAL, applied via the
DSN so every pooled connection inherits them (a post-open PRAGMA only
touches one connection); WAL moves to the DSN too. :memory: test DBs are
left as-is.

Tests: writer drains on Stop, Stop is idempotent, and the prune queries
keep newest-N per site / N logs on real SQLite. Full suite green under
-race.

internal/store/sqlite.go

@@ -2,6 +2,7 @@ package store
 
 import (
 	"database/sql"
+	"fmt"
 	"log"
 
 	_ "github.com/mattn/go-sqlite3"
@@ -10,13 +11,20 @@ import (
 type SQLiteDialect struct{}
 
 func NewSQLiteStore(path string) (*SQLStore, error) {
-	s, err := NewSQLStore("sqlite3", path, &SQLiteDialect{})
+	// Apply pragmas via the DSN so every pooled connection gets them — a
+	// post-open PRAGMA Exec only affects a single connection. WAL allows
+	// concurrent readers alongside the single writer goroutine; busy_timeout
+	// rides out brief lock contention; synchronous=NORMAL is durable under WAL
+	// and far faster than the FULL default. (:memory: is left untouched —
+	// these pragmas are no-ops or harmful for the in-memory test DB.)
+	dsn := path
+	if path != ":memory:" {
+		dsn = fmt.Sprintf("file:%s?_journal_mode=WAL&_busy_timeout=5000&_synchronous=NORMAL", path)
+	}
+	s, err := NewSQLStore("sqlite3", dsn, &SQLiteDialect{})
 	if err != nil {
 		return nil, err
 	}
-	if _, err := s.db.Exec("PRAGMA journal_mode=WAL"); err != nil {
-		log.Printf("WAL mode failed: %v", err)
-	}
 	return s, nil
 }