perf(agent): pattern/service catalog lists

[hoalongnatsu]

What does this PR do?

Adds server-side pagination to the agent catalog list endpoints (GET /api/agent/patterns, GET /api/agent/services) so they serve a bounded page plus a cheap total count instead of materializing the whole catalog on every request. Brings these two lists onto the same pattern the incidents/analyses lists already use.

Why?

On the Postgres backend, listPatterns/listServices went through catalog.Snapshot() — an unbounded SELECT over vs_patterns/vs_services plus a full JSON serialize and per-row readiness compute on every list load. This is the catalog twin of the incidents/analyses pagination already shipped. No linked issue — internal perf follow-up.

How to test

1. Run OSS with STORAGE_TYPE=postgres and generate enough varied logs across many services to learn more patterns/services than one page (or pass a small page_size).
2. GET /api/agent/patterns?page_size=5 → 5 rows, total = whole-catalog count, next_offset set on a full page; walk offset/page → non-overlapping, stable, gap-free windows; next_offset null on the last underfull page.
3. ?q=<substring> filters server-side (smaller total); re-fetching a page is deterministic (patterns fleet-count desc→id; services first_seen→name).
4. GET /api/agent/services?page_size=5 → paged envelope; services stays a name→facts map with grace fields; total = whole set.
5. page_size clamps to [1,5000] (default 1000); negative offset → 0.
6. UI Patterns/Services pages: header shows the whole-set total, Load more appends the next page, table sort works on loaded rows.

Type of change

• Bug fix (non-breaking)
• New feature (non-breaking)
• Breaking change (config / API / default behavior)
• Documentation only
• Refactor (no functional change)
• CI / build / chore

Checklist

• go test versus passes locally
• go vet versus is clean
• Code is gofmt
• Added or updated tests for the change
• Updated user-facing docs under src/ if behavior changes (internal admin API; no src/ doc change)
• Updated ROADMAP.md if this closes a roadmap item
• No secrets, tokens, or webhook URLs introduced in source / YAML
• No new third-party dependencies (or justified in the description)