Refactor: extract CDC EventBus into infra layer, unify event pipeline, add integration test harness

[ElioNeto]

Problem

Several architecture issues were identified during the v2.2.0 implementation:

1. EventBus lives in api layer, should be in infra

src/api/events.rs contains the EventBus CDC publisher — a general-purpose pub/sub mechanism. It imports from crate::infra::cdc but is itself in the api layer. This violates layering: the infra layer should own the event bus abstraction, and api should only wire it up.

Impact: Any non-API code that wants to subscribe to engine events (e.g., TUI, CLI, Notes) would need to depend on the API module.

2. CDC publisher chain is ad-hoc

The MultiPublisher in src/infra/cdc.rs solves the immediate need to fan out to EventBus + webhook, but it has no backpressure, no error isolation, and no monitoring. A single slow publisher blocks the entire chain synchronously.

3. TransactionManager uses std::sync::Mutex under actix-web

The TransactionManager in src/api/mod.rs protects active transactions with std::sync::Mutex<HashMap<u64, Transaction>>. The take/insert pattern for every txn_put/txn_delete is:

• High contention under load (serializes all transaction mutations)
• Race-prone (forgotten re-inserts leave dangling transactions)
• Not integrated with actix-webs timeout/cleanup — transactions can leak

4. API module is 1900+ lines

src/api/mod.rs has grown to ~1920 lines containing:

• Route registrations
• Handler functions
• Request/response types (Deserialize structs)
• Server startup logic (TLS, CDC, middleware)
• Transaction management
  This violates single-responsibility and makes testing hard.

5. No HTTP integration test infrastructure

Unlike the engine (which has extensive unit tests via #[cfg(test)]), there is zero test infrastructure for HTTP-level tests. No shared test helpers, no test server builder, no fixture system.

Proposed Solution

Phase 1: Move EventBus to infra

• Move EventBus from src/api/events.rs to src/infra/events.rs
• Implement CdcPublisher for EventBus there
• Add subscribe() method returning tokio::sync::broadcast::Receiver<CdcEvent>
• Re-export from src/infra/mod.rs
• Update API module to import from crate::infra::events

Phase 2: Create API test harness

• Create src/api/test_helpers.rs with:
  • fn test_server(engine: LsmEngine) -> impl Service — builds an actix-web test app
  • fn test_engine() -> (LsmEngine, TempDir) — creates engine with test config
  • Common fixture setup functions
• This enables writing integration tests as unit tests in api module or integration tests in tests/

Phase 3: Split api/mod.rs into modules

• src/api/routes.rs — all route registrations and handler functions
• src/api/types.rs — all request/response Deserialize structs
• src/api/server.rs — server startup, TLS, middleware wiring
• src/api/transactions.rs — TransactionManager and txn handlers
• src/api/mod.rs — re-exports and module declarations

Phase 4: Improve TransactionManager

• Use tokio::sync::RwLock<HashMap<u64, Transaction>> instead of std::sync::Mutex
• Add TTL-based cleanup for abandoned transactions
• Simplify API to avoid take/insert pattern

Acceptance criteria

• No functional changes — all existing tests pass
• EventBus can be imported from crate::infra::events
• Test harness can be used by new integration tests
• api/mod.rs is < 500 lines after splitting
• Transaction cleanup prevents leaks