Make SQLite max_connections configurable

[deepjoy]

Description

The SQLite connection pool in src/metadata/sqlite.rs currently hardcodes max_connections(5). This should be configurable, either per-bucket or as a global setting.

Location

• src/metadata/sqlite.rs:52

Details

The current code always creates a pool with 5 max connections:

let pool = SqlitePoolOptions::new()
    .max_connections(5)
    .connect_with(options)
    .await

This should be exposed as a configuration option (e.g., in config.toml) so operators can tune it based on their workload and deployment.

Acceptance Criteria

• Add a max_connections field to the config struct (with a sensible default of 5)
• Wire it through to SqlitePoolOptions::new().max_connections(...)
• Document the new config option