feat(logging): [4/4] DB connection-error logging middleware (database channel)

[turegjorup]

PR 4 of 4 — Structured logging

Final PR of the series. Base is feature/logging-phpstan-gate until the earlier PRs merge, then retargets to release/3.0.0.

Logs MariaDB connection-establishment failures from the application layer so operators without database access get a failure signal (ADR 011). Logging-only — no reconnect/retry.

What this PR does

• database channel — added to monolog.yaml and a prod per-channel handler (LOG_LEVEL_DATABASE, same LOG_LEVEL fallback, LOG_PATH destination).
• ConnectionErrorMiddleware + ConnectionErrorDriver — a DBAL driver middleware wrapping connect(). It reads the raw driver error code, because a middleware sits below DBAL's exception conversion, which does not classify 1040 (too many connections), 1203, 2003 (connection refused), etc. as ConnectionException. Fires for every SAPI (web, CLI, Messenger) regardless of whether app code later swallows the exception.
  • Connection pressure / unreachability (1040, 1203, 1226, 2002, 2003, 2005) → critical; other connect failures (e.g. 1045 credentials) → error.
  • Logs event=db.connection_error, db.error_code, db.sqlstate, db.host (host only — never password/DSN), and the exception under the exception key.
  • The original exception is rethrown unchanged.
• Wiring — registered with the explicit doctrine.middleware tag (doctrine-bundle collects tagged services in MiddlewaresPass; implementing the interface alone isn't enough), bound to @monolog.logger.database. Confirmed present on the default connection via debug:container --tag=doctrine.middleware.
• Docs — docs/logging.md gains the database channel and the event=db.connection_error shape.

Mid-query drops (2006/2013) are out of scope (establishment only), per ADR 011.

Verification

• composer code-analysis — green
• composer coding-standards-check + env-coverage — green
• composer test — 180 tests, green (added ConnectionErrorDriverTest: a stub driver throws Driver\Exception with code 2003→critical / 1045→error; asserts channel, level, db.* keys, and no secret-bearing keys)
• cache:clear --env=prod — handler + middleware wiring resolve

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