feat: add cancel reservation handler and background cleanup

Add CancelReservationHandler (DELETE /api/bookings/reserve) to release authenticated user's active reservation. Register route in main.go. Add background goroutine for periodic reservation cleanup using CleanupOldReservations. Add idx_time_blockers_created_at index and extend anon cleanup to cover edit_request reservations in init-script.sql.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

init-scripts/init-script.sql

@@ -604,6 +604,7 @@ CREATE TABLE time_blockers (
 CREATE INDEX idx_time_blockers_start_time ON time_blockers(start_time);
 CREATE INDEX idx_time_blockers_cron ON time_blockers(cron_expression) WHERE cron_expression IS NOT NULL;
 CREATE INDEX idx_time_blockers_created_by ON time_blockers(created_by);
+CREATE INDEX idx_time_blockers_created_at ON time_blockers(created_at);
 CREATE INDEX IF NOT EXISTS idx_time_blockers_desc_trgm ON time_blockers USING GIN (description gin_trgm_ops);
 
 -- =======================================
@@ -918,7 +919,8 @@ BEGIN
     UPDATE time_blockers
     SET description = NULL
     WHERE created_by = target_id
-      AND description LIKE 'RESERVATION:user:%';
+      AND (description LIKE 'RESERVATION:user:%'
+        OR description LIKE 'RESERVATION:edit_request:%');
 
     -- Scrub notes on booking_edit_requests made by this user (free-text PII)
     UPDATE booking_edit_requests