feat(channels): P2 — clustering & editorial collections

[jung-thomas]

P2 — Clustering & Editorial Collections (external SAP channels integration)

Implements Phase 2 of the external channels integration spec (docs/superpowers/specs/2026-09-04-external-channels-integration-design.md), adding curator-reviewed editorial collections that group published channels and lead the /channels directory landing.

What's included (7 tasks, spec §5.2/§8/§10/§13/§15)

1. Model — ChannelCollections + ChannelCollectionItems entities (reuse existing AuthoringStatus enum), @cds.persistence.journal, regenerated csn + migration tables.
2. Public feed — GET /build/channel-collections, hard-gated on isPublished && authoringStatus === 'REVIEWED', broken-link item filtering (P1 parity).
3. Admin service — ChannelCollections (draft-enabled) + ChannelCollectionItems projections on AdminService.
4. Admin FE app — app/admin/channel-collections/ Fiori Elements app + shell registration (nav, prefix, manifest regenerated).
5. LLM seed pass — srv/lib/channels/seed-collections.js + npm run seed-collections; idempotent, preserves REVIEWED, writes drafts as AI_SEEDED; AI SDK lazily imported (off srv-qa boot path).
6. Landing — collections lead the /channels directory; baked hugo/data/channel_collections.json + island section, degrades cleanly when empty.
7. e2e nudge — tolerant /channels collections spec (self-skips without SMOKE_BASE_URL).

Scope boundary

P3 (ChannelTopicMap) and P4 (ChannelSubmissions) are explicitly out of this PR.

Testing

• 20 P2 unit specs pass (model, feed, admin service, admin app, seed, island).
• check-cds-build-staging and check-srv-qa-cp-list gates exit 0.
• Un-reviewed drafts never render publicly (feed REVIEWED-gate verified end-to-end).

Review notes

Built via subagent-driven development (fresh implementer + spec/quality gate per task, broad final review). Final review caught + fixed a Hugo data-key mismatch (hyphen vs underscore filename) that would have silently hidden all collections — now channel_collections.json matches .Site.Data.channel_collections.

Not deployed — deploy is a separate explicit step.