feat: adaptive retry with configurable backoff strategies

[deepjoy]

Summary

• Add configurable backoff strategies (Constant, Linear, Exponential, ExponentialJitter) and per-task-type RetryPolicy registration via SchedulerBuilder::executor_with_retry_policy()
• Persist per-task max_retries in the schema (migration 008_retry_backoff.sql) so retry behavior is deterministic across restarts and policy changes
• Compute backoff delays in the store fail path using the existing run_after column — zero new scheduling infrastructure needed
• Support executor-signaled retry delays via TaskError::retryable("...").retry_after(Duration) for honoring upstream Retry-After headers
• Add HistoryStatus::DeadLetter for tasks that exhaust retries (distinct from permanent Failed), with Scheduler::dead_letter_tasks() query and Scheduler::retry_dead_letter() for manual resubmission
• Emit SchedulerEvent::DeadLettered and add retry_after field to SchedulerEvent::Failed
• Update migration guide (docs/migrating-to-0.4.md) with all breaking changes and new APIs
• Fix all 23 pre-existing rustdoc warnings (private intra-doc links, unresolved paths, redundant explicit targets)

Fixes #13

Breaking changes

• SchedulerEvent::Failed gains retry_after: Option<Duration> field — exhaustive matches must be updated
• SchedulerEvent::DeadLettered new variant — exhaustive matches must add an arm
• HistoryStatus::DeadLetter new variant — exhaustive matches must add an arm
• TaskError gains retry_after_ms: Option<u64> field — struct literal construction must include it (constructors unaffected)

Test plan

• Unit tests for BackoffStrategy::delay_for() across all strategies (constant, linear, exponential, jitter) including clamping and edge cases
• Unit tests for registry policy storage (register, lookup, missing returns None)
• Store tests: migration applies cleanly, max_retries round-trips, NULL backward compat
• Store tests: backoff delay sets correct run_after for Constant and Exponential strategies
• Store tests: executor retry_after override takes precedence over strategy
• Store tests: exhausted retries produce dead_letter status; non-retryable produces failed
• Store tests: dead_letter_tasks() returns only dead-lettered records
• Integration test: per-type policy overrides global default
• Integration test: exponential backoff delays redispatch until delay elapses
• Integration test: Failed event includes correct retry_after duration
• Integration test: NULL max_retries falls back to global default
• Integration test: retry_dead_letter() resubmits with reset retry count
• All 46 integration tests pass
• All 167 unit tests pass
• All benchmarks pass with no regressions
• cargo doc builds with zero warnings under RUSTDOCFLAGS="-D warnings"