Implement time off management (Issue #3)

Add full time-off lifecycle: create/edit/delete with shift conflict
detection, auto-removal from conflicting shifts with admin notification,
shift restoration on admin delete, and hard block on assigning volunteers
with approved time off to shifts.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/db/schema.go

@@ -134,5 +134,15 @@ var statements = []string{
     FOREIGN KEY (volunteer_id) REFERENCES volunteers(id) ON DELETE CASCADE,
     INDEX idx_instance_id (instance_id),
     INDEX idx_volunteer_id (volunteer_id)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci`,
+	`CREATE TABLE IF NOT EXISTS time_off_removed_shifts (
+    id            INT AUTO_INCREMENT PRIMARY KEY,
+    time_off_id   INT NOT NULL,
+    instance_id   INT NOT NULL,
+    volunteer_id  INT NOT NULL,
+    FOREIGN KEY (time_off_id) REFERENCES time_off_requests(id) ON DELETE CASCADE,
+    FOREIGN KEY (instance_id) REFERENCES shift_instances(id) ON DELETE CASCADE,
+    FOREIGN KEY (volunteer_id) REFERENCES volunteers(id) ON DELETE CASCADE,
+    INDEX idx_time_off_id (time_off_id)
 ) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci`,
 }