Add context cancellation support to database operations

- Add context-aware versions of all read operations in the database
- Update handlers to use context from HTTP requests
- Allows database queries to be cancelled when HTTP requests timeout
- Prevents database operations from continuing after client disconnects

internal/database/slowquery.go

@@ -19,6 +19,7 @@ func logSlowQuery(logger *logger.Logger, query string, start time.Time) {
 }
 
 // queryRow wraps QueryRow with slow query logging
+// nolint:unused // kept for consistency with other query wrappers
 func (d *Database) queryRow(query string, args ...interface{}) *sql.Row {
 	start := time.Now()
 	defer logSlowQuery(d.logger, query, start)