Tags: tag-filtered feed (`GET /api/messages?tag=`)

[Adron]

Part of #27 — Tags: tagging posts, autocomplete, trending and tag-filtered feeds

• Tapping a tag on a message card opens a feed filtered to that tag via the tag query
  parameter.
• The tag feed is a normal paged feed — it should share the feed implementation (including the
  cursor-pagination work) rather than forking it.
• Deep-link support so a tag URL from elsewhere opens the right screen.

Tests: tap → filtered query; paging works on the filtered feed.

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Filed from the parity review of feature/parity-wip @ 14d7795 against interlinedlist.com (OpenAPI /api/openapi.json, 233 paths) and the public help centre.

[Adron]

Implemented in #122 (branch issue/29-tag-feed). Closing here — review and any follow-up happens on the PR.

The "share the feed implementation rather than forking it" requirement drove the interesting part. Sharing the ViewModel and repository was easy; the Room cache was not. The main feed and a tag feed routinely contain the same message, and a message row cannot hold two positions — so with feedOrder on the message, the second feed to load would have silently stolen rows from the first. Feed membership now lives in a feed_entry(feedKey, messageId, position) join table over shared message rows.

The deep-link URL was established, not guessed. /tag/lists, /tags/lists, /tags, /explore and /trending all 404, and /messages is Direct Messages rather than the feed. The web's own tag chip (found in its JS chunk) links to "/?tag=".concat(encodeURIComponent(e)), and https://interlinedlist.com/?tag=lists does serve a filtered feed. So the canonical form is https://interlinedlist.com/?tag=<encoded>.

One product decision for you: an Android intent filter cannot match on a query string, so catching /?tag=… means registering https://interlinedlist.com/ — which also claims plain homepage links. They land on the app's normal start destination showing the same feed, so nothing breaks, but it is a wider claim than the feature strictly needs. Say if you would rather not claim it.