Performance + reliability hardening and clippy cleanup

[johnny-emp]

Summary

• harden event log reads/writes with max event and batch byte limits
• make migration apply/rollback metadata updates transactional
• improve SQLite read concurrency by using read pool in query/query_async and switching to BEGIN IMMEDIATE
• optimize preflight cache eviction from O(n) scan to queue-based eviction
• preserve event order in batched handler processing
• wire signer-backed provider path for on-chain backup registration
• clean up compiler/clippy warnings across tests, benches, and examples

Validation

• cargo check --workspace --all-targets
• cargo check -p azoth --features onchain
• cargo clippy --workspace --all-targets -- -D warnings
• targeted tests for sqlite read pool, file log, and event handler batching