Refactor log DB into LogWriter interface

[rasmusrygaard]

Why

This prepares feedback log capture for a future remote app-server hook sink without changing the current local SQLite upload path. The important boundary is now intentionally small: a log sink is a tracing Layer that can also flush entries it has accepted.

That keeps the existing SQLite implementation simple while giving the upcoming gRPC sink a place to fit beside it. SQLite and gRPC have different worker/write semantics, so this PR avoids introducing a shared buffered-sink abstraction and instead lets each LogWriter own the buffering mechanics it needs.

What Changed

• Added LogSinkQueueConfig with the existing local defaults: queue capacity 512, batch size 128, and flush interval 2s.
• Added LogDbLayer::start_with_config(...) while preserving LogDbLayer::start(...) and log_db::start(...) defaults.
• Introduced the LogWriter trait as the minimal shared interface: tracing_subscriber::Layer plus flush().
• Made LogDbLayer implement LogWriter.
• Kept tracing event formatting inside LogDbLayer; it still creates one LogEntry per tracing event before queueing it for SQLite.
• Kept normal event capture best-effort and non-blocking via bounded try_send.

Behavior Notes

This does not change the SQLite schema, retention behavior, /feedback/upload, or Sentry upload behavior. Normal log events still drop when the queue is full; explicit flush() still waits for queue capacity and receiver processing before returning.

Verification

• cargo test -p codex-state log_db
• cargo test -p codex-state
• just fix -p codex-state

The added tests cover configured batch-size flushing, configured interval flushing, queue-full drops, and the flush barrier semantics.

[pakrym-oai]

There are a lot of new abstractions there. Can we introduce a single trait LogWriter that implements tracing_subscriber::Layer and flush and make LogDb layer implement that?