chore: purge identified terraform modules via dbpurge (#28802) - #28816

Cherry-pick of #28802 (e2a856d42b), matching #28810 for release/2.37.

Deletes cached Terraform module archives ingested during the identified window and clears the template version references to them. Runs from dbpurge rather than a migration, because migrations cannot be backported: the version table records a single high-water mark, so a migration cherry-picked here would cause later upgrades to skip every migration in between.

Conflict resolution

The commit did not apply cleanly. This branch predates the chat search work on main, so the incoming hunks carried unrelated context that was dropped:

• dbpurge.go: took only the module cache block, the ranModuleCachePurge latch, the window constants, the identified_module_files log field and metric, and the identifiedModuleCachePurged instance field. This branch has the chat_messages.search_tsv backfill but not the stale reindex added in fix(site): clarify chat model permission labels (#28579) #28585, so the reindex loop, staleDrained, and chatSearchStaleDrained were dropped.
• dbpurge_test.go: took TestDeleteIdentifiedModuleCacheFiles. In the two TestMetrics mock setups, added only the DeleteCachedModuleFilesCreatedBetween expectation, dropping ReindexStaleChatMessagesSearchTsv.
• Generated files (querier.go, queries.sql.go, dbmetrics, dbmock, and the dbauthz stub) were reset to the branch state and regenerated from queries/files.sql, rather than taking the diff from main. Taking main's versions would have introduced methods for queries that do not exist on this branch.

Testing

coderd/database/dbpurge and TestMethodTestSuite in coderd/database/dbauthz pass against Postgres. make gen is clean.

Pre-commit hooks were skipped on the final commit at the author's request after the checks above had already been run; make gen, build, and lint were verified separately during conflict resolution.

Opened by Coder Agents on behalf of @Emyrk.