refactor(tracker-core): split persistence traits by context

[josecelano]

Goal

Decompose the monolithic Database trait into four focused context traits while keeping Database as the unified driver contract, and write an ADR to record the decision.

Background

packages/tracker-core/src/databases/mod.rs defines a single Database trait with 19 methods covering four unrelated concerns: schema management, torrent metrics, whitelist, and authentication keys. This makes the trait long and conflates distinct responsibilities in one place.

Two options were considered:

1. Replace Database with four independent traits — consumers hold Arc<dyn WhitelistStore> etc. directly. Clean interface segregation, but it loses the single place that tells a new driver implementor exactly what to build, and it changes every consumer at once.

2. Keep Database as an aggregate supertrait (chosen) — the four narrow traits exist independently; Database is defined as:

   pub trait Database:
       Sync + Send + SchemaMigrator + TorrentMetricsStore + WhitelistStore + AuthKeyStore {}

   A blanket impl means any type that implements all four narrow traits automatically satisfies Database. Existing consumers (Arc<Box<dyn Database>>) are untouched.

Tasks

• Write ADR: docs/adrs/<timestamp>_keep_database_as_aggregate_supertrait.md
• Introduce SchemaMigrator trait in databases/schema.rs
• Introduce TorrentMetricsStore trait in databases/torrent_metrics.rs
• Introduce WhitelistStore trait in databases/whitelist.rs
• Introduce AuthKeyStore trait in databases/auth_keys.rs
• Introduce Database aggregate supertrait + blanket impl in databases/database.rs
• Update SQLite driver to implement the four narrow traits
• Update MySQL driver to implement the four narrow traits
• Update mod.rs to declare new submodules and re-export traits/mocks

Acceptance Criteria

• ADR is written and added to docs/adrs/index.md.
• Four narrow traits exist in separate files under databases/.
• Database is an empty aggregate supertrait with a blanket impl.
• Both drivers (Sqlite, Mysql) compile through the blanket impl with no manual impl Database for <Driver> block.
• No existing consumer file (persisted.rs, downloads.rs, etc.) is changed.
• #[automock] is on the four narrow traits; MockDatabase is removed.
• No behavior change — existing tests pass without modification.
• cargo test --workspace --all-targets passes.
• linter all exits with code 0.

References

• Part of EPIC: Overhaul persistence #1525
• Spec file: docs/issues/1525-04-split-persistence-traits.md
• Reference PR: feat: rework persistence for async sqlx backends and PostgreSQL support #1695
• Reference implementation branch: josecelano:pr-1684-review
• packages/tracker-core/src/databases/mod.rs — current monolithic Database trait

[josecelano]

The new trait names were proposed in this PR by @DamnCrab. It was planned in the original Overhaul persistence EPIC