Implement the SQLite backend

[niteshpurohit]

Summary

Implement: the SQLite backend as a local durable backend option.

Scope

• deliver the backend class, queue-store implementation, runtime wiring, and directly-coupled tests for this capability
• position SQLite as a local/runtime-developer choice rather than a shared/server deployment backend
• cover the happy path, important failure paths, and boundary conditions that are part of the SQLite backend itself
• add or consolidate the backend-specific unit, integration, and end-to-end validation needed to prove the intended guarantees deterministically enough for CI use
• implement the persistence support this backend needs for jobs, retries, dead-letter handling, schedules, workflow state, audit-relevant history, and Kaal-managed recurring schedule behavior within its intended local-only support boundary
• update only the documentation, examples, caveats, and troubleshooting guidance that are tightly coupled to the SQLite backend implementation

Not In Scope

• broad docs sweeps, cross-milestone cleanup, or unrelated platform work
• presenting SQLite as a multi-node or ephemeral-server durability solution
• redefining the shared backend contract already established elsewhere

Acceptance Criteria

• the SQLite backend works end-to-end for supported local scenarios in this milestone
• important error, recovery, and integration paths are covered by tests
• the SQLite backend implements the required persistence surface for jobs, schedules, workflows, audit-relevant history, and Kaal-backed recurring schedule behavior within its local-only posture
• public behavior, caveats, and unsupported server-style cases are clear from the resulting implementation and tightly coupled docs