feat(tap): dynamic collection filters and multiple signal collections

[trezy]

Summary

Adds endpoints for dynamically updating collection filters at runtime, eliminating the need to restart Tap to add new collections.

Use case

I'm building HappyView, a lexicon-driven AppView software. I'm trying to leverage existing software in the Atmosphere that solves AppView responsibilities, such as Tap. The Tap server works well, except that we have to restart the whole server to add new collections.

This PR makes those collections dynamic, so they can be updated via an endpoint. This allows HappyView (and any other software leveraging Tap) to update the collections without further user intervention (i.e. restarting the Tap server).

New Endpoints

Path	Description
GET /collection-filters	Get current collection filters
PUT /collection-filters	Update collection filters at runtime
GET /signal-collections	Get current signal collections
PUT /signal-collections	Update signal collections at runtime

All endpoints accept/return { "collections": ["nsid1", "nsid2"] }.

Changes

• signal-collection string flag replaced with signal-collections string slice (backwards-compatible; still reads TAP_SIGNAL_COLLECTION env)
• Collection filters and signal collections are now protected by sync.RWMutex for safe concurrent read/write
• TapServer receives a reference to Resyncer so filter updates propagate to both firehose and resync workers
• Pluralised flags that are no longer singular

Test plan

• Verify TAP_SIGNAL_COLLECTION env still works (backwards compat)
• Verify TAP_SIGNAL_COLLECTIONS env works with comma-separated values
• PUT /collection-filters updates firehose and resyncer filtering in real time
• PUT /signal-collections updates repo discovery in real time
• Existing tests pass (go test ./cmd/tap/...)