Replaced the raw db.Exec call in createTaskQueueSchema with migrateDB,
matching the pattern used by createKVStoreSchema. This enables version-tracked
schema migrations via SQLite's PRAGMA user_version, allowing future schema
changes to be appended incrementally. Also renamed cleanupInterval to
taskCleanupInterval to resolve a redeclaration conflict with host_kvstore.go.
Add input validation (queue name length, payload size limits), extract
status string constants to eliminate raw SQL literals, make CreateQueue
idempotent via upsert for crash recovery, fix RetentionMs default check
for negative values, cap exponential backoff at 1 hour to prevent
overflow, and replace manual mutex-based delay enforcement with
rate.Limiter from golang.org/x/time/rate for correct concurrent worker
serialization.
