chore(backend): harden sqlite concurrent writes and add stress tests

[hammond01]

Summary

• replace fragile MAX(row_id)+1 flow with safer insert/upsert + rowid backfill in SQLite repositories
• add lock-aware retry with short backoff for SQLite lock/busy contention during writes
• make activity log writes idempotent via ON CONFLICT(id) DO UPDATE
• expand SQLite repository integration tests with concurrent insert stress coverage for captures, items, and activity logs

Why

SQLite shared-cache writes can transiently fail under concurrent writes (SQLITE_LOCKED_SHAREDCACHE). This PR improves resilience and ensures row id assignment remains stable under load.

Validation

• backend tests: ./gradlew test

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