feat(stats): track ROM downloads and add an admin statistics tab

[bikeborb]

Adds a download_events audit log plus denormalized per-ROM counters, so an operator can see what is actually being downloaded and identify content nobody uses.

• New download_events table: user, ROM, platform, source, kind, byte count, client IP and user agent, with name snapshots so a row survives the deletions it exists to inform.
• roms.download_count / roms.last_downloaded_at, incremented in SQL so concurrent downloads of the same ROM cannot lose an increment.
• Recording hooks on the single-ROM, single-file and bulk-zip download endpoints. Inline media (soundtracks, manuals) is excluded, so opening a game page does not inflate its count.
• HybridAuthBackend records how a request authenticated, which is what distinguishes a WebUI download from an API client.
• Admin-gated /api/stats/downloads endpoints and a Downloads tab in the existing Administration page.
• Optional retention via DOWNLOAD_EVENTS_RETENTION_DAYS, off by default.

Recording is best-effort and never fails a download.

Description
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Checklist
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• I've tested the changes locally
• I've updated relevant comments
• I've assigned reviewers for this PR
• I've added unit tests that cover the changes

Screenshots (if applicable)

[bikeborb]

Following the feedback that this diff is too large, and after re-reading
CONTRIBUTING.md, I've filed #4089 to discuss the feature first and split
the work into two reviewable halves:

• Backend: 23 files, the event table, migration, API and tests
• Frontend: 57 files, but 48 of those are locale key copies and generated
  API types; the hand-written UI is 9 files

I'll open those once there's a steer on #4089, so please don't spend
review time on this PR. Happy to close it now or leave it as context.