feat: force-acquire after TTL when ExpireGracePeriod is set

[mashiike]

This PR introduces an optional ExpireGracePeriod setting for setddblock that allows a lock to be forcibly reacquired after its TTL has expired plus a configurable grace period.

Key changes

• CLI support: adds --expire-grace-period flag to setddblock CLI and the corresponding ExpireGracePeriod field to Options oai_citation:0‡patch-diff.githubusercontent.com.
• Forced reacquisition logic: implements a new acquireLock method in DynamoDBLocker that, when ExpireGracePeriod > 0, checks if the elapsed time since the lock’s ExpireTime exceeds the grace period. If so, it sets PrevRevision and retries AcquireLock to take over the lock oai_citation:1‡patch-diff.githubusercontent.com.
• TTL tracking: adds an ExpireTime field to lockOutput and reads the ttl attribute from DynamoDB so the locker knows when a lock has actually expired oai_citation:2‡patch-diff.githubusercontent.com.
• Test coverage: adds ttl_grace_test.go to verify that locks are force‑acquired when past TTL+grace and not otherwise oai_citation:3‡patch-diff.githubusercontent.com oai_citation:4‡patch-diff.githubusercontent.com.
• Time control for testing: switches from time.Now() to flextime.Now() and related helpers to make time-based tests deterministic oai_citation:5‡patch-diff.githubusercontent.com.

If ExpireGracePeriod <= 0, automatic reclaim is disabled and the system continues to depend on DynamoDB TTL cleanup oai_citation:6‡patch-diff.githubusercontent.com.