db: make outbox inserts atomic

[OllieinCanada]

Summary

Make Sashiko's notification outboxes idempotent under concurrent review completion.

• replace separate deduplication lookups and inserts with one conditional insert per outbox path
• cover review email, Patchwork status update, and Patchwork failure notification queues
• add deterministic tests that release 32 concurrent writers for each logical notification and verify exactly one row is stored

Root cause

Each outbox writer first queried for an existing row and then issued a separate insert. Two review tasks completing together could both observe that no row existed before either insert ran, allowing duplicate external notifications to be queued.

The code described these paths as duplicate-safe, but the invariant was not enforced atomically at the persistence boundary.

Design

Each writer now uses a single INSERT ... SELECT ... WHERE NOT EXISTS statement. SQLite/libSQL serializes the conditional check and write as one statement, closing the check-then-act window without a schema migration or destructive cleanup of historical outbox rows.

The existing logical deduplication keys remain unchanged:

• review email: patch ID
• Patchwork update: message ID, API URL, and context
• Patchwork notification email: recipient, subject, and reply target

Validation

• standalone rustfmt --check passes for src/db.rs
• git diff --check passes
• direct SQLite concurrency probe passes for all three statements with 32 separate writers
• Rust concurrency tests are included for all three outbox paths
• local cargo test and make check-pr stop before compiling Sashiko because aws-lc-sys rejects the installed GCC 9.4 compiler; GitHub CI is expected to provide the authoritative Rust, clippy, and test results