feat: add redis, mongodb and sqlx session stores

[chrislearn]

Summary

• Three new pluggable session stores behind cargo features: redis, mongodb, and sqlx (with sub-features sqlx-postgres / sqlx-sqlite / sqlx-mysql).
• All SQLx stores use sqlx::Pool for connection pooling; RedisStore uses ConnectionManager; MongoDbStore relies on the official driver's built-in pool.
• Bilingual (en/zh) usage docs under docs/ and ignored integration tests for every backend.

Stores

Store	Feature	Notes
RedisStore	redis	TTL via SET ... EX, configurable key prefix
MongoDbStore	mongodb	initialize() creates a TTL index on expires_at
SqlxPostgresStore	sqlx-postgres	migrate() / cleanup() helpers
SqlxSqliteStore	sqlx-sqlite	same API
SqlxMySqlStore	sqlx-mysql	same API

SQLx stores share a portable schema (expires stored as BIGINT unix seconds) so no extra sqlx type-conversion features are required.

Test plan

• cargo check --features redis
• cargo check --features mongodb
• cargo check --features sqlx-postgres,sqlx-sqlite,sqlx-mysql
• cargo test --features redis,mongodb,sqlx-postgres,sqlx-sqlite,sqlx-mysql --no-run
• Run ignored integration tests against live services (see docs/en/stores.md for commands)

🤖 Generated with Claude Code

[chrislearn]

Opened against the wrong repo by mistake — re-creating against salvo-rs/saysion.