feat: paginate subscription group memberships

[kapdon]

Summary

The subscription group manager currently waits for the complete membership projection before displaying a page. Add an API that filters, counts and paginates in PostgreSQL, then loads memberships and repairs avatars only for the returned channels.

Required by TypeType-Frontend #32, following the pagination review. Refs TypeType #172.

• GET /subscriptions/group-memberships/page: zero-based page, limit (1–100), literal substring search, named/ungrouped/inverted membership filters, matching total and account-wide subscription/ungrouped counts. Order is lowercase name, then canonical URL. Offset pages reflect current results; they are not a cross-request snapshot.
• POST /subscriptions/group-memberships/lookup: read-only refresh of explicitly selected subscriptions retained across pages. Reuses the existing 500-URL, 2048-character and 1 MiB request limits, canonicalizes/deduplicates URLs and omits missing or foreign subscriptions.
• Retain the legacy full-array endpoint for existing clients. Aggregate group counts in SQL instead of loading every membership row.
• Use correlated membership existence checks for included, excluded and ungrouped filters. Normalize both the search input and stored values in PostgreSQL while preserving literal wildcard escaping.
• Document both endpoints in OpenAPI and add route/service tests covering 25,000 channels with 112,500 memberships, stable page boundaries, Unicode search and escaping, all membership filters, ownership, missing groups, authentication and lookup limits.
• Let tests use an explicitly supplied TEST_DATABASE_URL, with pg_trgm installed in the shared test search path for repeat runs. The default Testcontainers setup remains available.

Validation

./gradlew test shadowJar validateOpenApi check with JDK 25 and an isolated local PostgreSQL 16 database. 1,251 passed, 3 skipped against dev at 302b76e5; build, OpenAPI validation and coverage checks passed. Test libraries emit existing native-access/Unsafe runtime warnings.

Companion frontend behavior was checked in Chromium with a 150-channel fixture: bounded page requests, retained selections/drafts, membership search and inversion, last-page adjustment, and failed selection-refresh recovery. Firefox/WebKit and a browser session against the running Kotlin server remain unverified.

No new dependencies or database migrations. Each commit has fewer than 300 insertions.

[Priveetee]

Hey @kapdon, thx again for this PR! The SQL pagination and the test coverage look exactly like what we need for large subscription lists! :)

Before I start the full review, the branch needs an update: dev now uses the modular server layout, so the PR currently conflicts with the old single-module paths.

Mainly, the files need to move to their owning modules:

• SubscriptionMembershipPage.kt -> server-core
• SubscriptionMembershipPageRoutes.kt -> server-http
• SubscriptionGroupQueries.kt, SubscriptionMembershipFilter.kt and SubscriptionMembershipPageService.kt -> server-services
• the tests -> the matching module source sets
• TestDatabase -> the one from server-db test fixtures

No pagination changes expected here. If it’s easier for u, I can handle that update myself, no worries at all! Just tell me what u prefer. Once the branch is clean against dev, I’ll review it :)

[kapdon]

ok should be good now

[Priveetee]

Thank you for the tremendous amount of work you put into this PR! I really appreciate the effort behind the pagination and membership changes :)

I pushed a small follow-up to improve search performance and concurrent reads: trigram indexes for subscription search, shared locks for membership reads while preserving exclusive write locks, and read-only avatar enrichment. I also added tests for search scale, reader/writer concurrency, and avatar enrichment.

I ran the full test suite, check, and shadowJar; the PR’s build, coverage, and OpenAPI checks are all passing. LGTM from my side! I’ll merge it into dev :)