fix(api-rest): clear database routing context across request lifecycles

[srreejitbarat]

When a controller throws or starts asynchronous handling, postHandle is skipped and the database ID can remain on a reused servlet thread. A later whitelist/admin request can inherit that route.

This clears stale context before route validation, performs synchronous cleanup in afterCompletion, and releases context at afterConcurrentHandlingStarted. Async redispatch establishes its own route; worker services continue selecting JDBC/transaction templates with explicit dbId. Invalid-route diagnostics no longer log request paths or database IDs.

Refs #958.

Validation — 16 October 2026:

• Full build succeeded on the exact PR head 33109c6: 765 tests reported, 748 passed, 17 skipped, zero failures/errors.
• The fork ran the unchanged upstream command with Liberica JDK 21 and the Maven wrapper: ./mvnw --quiet -Drevision=1.6.6-SNAPSHOT --no-transfer-progress -B verify --file pom.xml
• Full run and report artifact: https://github.com/srreejitbarat/db2rest/actions/runs/38852092709/job/116815349728
• The previous upstream run failed the same 41 existing tests as PR fix: bind and release JDBC arrays on the statement connection #969: 40 SQL-template GET routes and one unsupported SQLite PATCH URL. Upstream PR fix: restore SQL template routing and PostgreSQL coverage #967 fixes those failures and was merged into this branch from master 4b182bf. The contribution diff is unchanged.
• Earlier focused verification: 51 tests passed with zero skips, including 19 lifecycle cases, 3 SQLite routing cases, 9 metadata-controller cases and 20 existing PostgreSQL cases: https://github.com/srreejitbarat/db2rest/actions/runs/38760664089

The new upstream workflow requires maintainer approval: https://github.com/9tigerio/db2rest/actions/runs/38851879422 . The successful fork build does not replace that required check.

Draft pending maintainer review and upstream workflow approval. The timeout regression models a deferred timeout result and redispatch rather than a real container timer. The context-consumer inventory and reproduction notes are in design/database-context-lifecycle.md.

AI assistance was used for implementation, tests and documentation.