fix: drop transaction from station_battle flush to relieve DB contention

[jfberry]

Problem

Production on main is flooding with getOrCreateStationRecord: context deadline exceeded.

flushStationBattleBatch (added in #353) is the only write-behind flush wrapped in a transaction — every other flush (flushPokestopBatch, flushGymBatch, flushStationBatch, …) is a single autocommit INSERT … ON DUPLICATE KEY UPDATE. Wrapping the batch upsert and the obsolete-prune DELETE … WHERE station_id IN (…) AND bread_battle_seed NOT IN (…) in BeginTxx … Commit holds both statements' InnoDB locks (incl. next-key/gap locks on ix_station_battle_station_end) until commit.

Under concurrent overlapping batches (up to 50 write-behind workers × 50-station batches), those transactions block each other (lock-waits up to innodb_lock_wait_timeout, or 1213 deadlock-retries) and pin connections on the shared GeneralDb pool (PokemonDb and GeneralDb are the same handle). That starves the synchronous station reads (loadStationFromDatabase + the new battle hydrate) which run under the station entity lock with only the GMO's 5s context — surfacing as the context deadline exceeded flood.

This is not a Go mutex deadlock (the battle cache is a lock-free xsync.MapOf; the enqueue is non-blocking) — it's DB-level lock/connection contention.

Fix

Drop the transaction. Run the upsert as a single autocommit NamedExecContext (matching every other flush) and the prune DELETE as a separate autocommit ExecContext, upsert-first. Each statement releases its locks immediately, the cross-statement deadlock shape is gone, and the pool connection is returned between statements.

No query text changed — only the BeginTxx/Commit/Rollback wrapper is removed.

Consistency tradeoff (acceptable, self-healing)

Losing atomicity between the two statements means:

• A reader in the sub-ms gap can see the new battles plus a few about-to-be-pruned superseded ones — over-inclusive only (the reader filters battle_end > now, and the upsert runs first); the next hydrate corrects it.
• A crash between the statements leaves a few obsolete rows, removed by the next flush or by expiry.

Test Plan

• go build -tags go_json ./..., go vet ./..., go test ./... — all green (incl. all station-battle tests; buildDeleteObsoleteStationBattlesQuery query-building unchanged).
• Deploy and confirm the getOrCreateStationRecord: context deadline exceeded flood stops under load — the contention relief is operational and can only be confirmed against a live DB. If it persists, the dominant factor may be raw write volume rather than lock-hold time (next levers: separate read pool, larger max_pool, move battle hydrate out from under the entity lock).

🤖 Generated with Claude Code

[jfberry]

Probably not needed