Fix repo selector performance and search quality by replacing live GitHub paging on every query with a user-scoped cached repo index

Summary: Fix repo selector performance and search quality by replacing live GitHub paging on every query with a user-scoped cached repo index. The cache will be keyed by user and installation so search stays fast without widening repository visibility across users.

Context:

• The current selector flow is repo-selector(-compact) -> useInstallationRepos -> /api/github/installations/repos -> listUserInstallationRepositories.
• apps/web/lib/github/installation-repos.ts currently pages GET /user/installations/{installationId}/repositories with the user's OAuth token and filters in memory.
• That implementation is slow because each search can require multiple sequential GitHub requests.
• Search quality is poor because paging stops once enough matches are found, so results depend on early pages rather than the full accessible repo set.
• The current security boundary is correct: GitHub computes the intersection of app installation access and that specific user's repo access. A shared installation-level cache would break that boundary.
• apps/web/components/repo-selector-compact.tsx additionally re-sorts results client-side, which would override any future server-side ranking.
• Installation webhook handling already updates installation metadata in apps/web/app/api/github/webhook/route.ts, which is the right place to mark repo caches stale when access changes.

System Impact:

• Part of the system changing: repo discovery for the selector, not installation ownership or GitHub auth.
• Source of truth before: live GitHub responses per search request.
• Source of truth after: a per-user cached snapshot of repos derived from GitHub, refreshed from the same user-scoped GitHub endpoint.
• New state/invariants:
  • Repo cache rows must always belong to a single userId and installationId.
  • Reads must only return repos after verifying the installation belongs to session.user.id.
  • Cache refresh must never populate rows from an app-scoped token or for another user.
  • Installation metadata needs a refresh/staleness signal so the API can distinguish warm, stale, and missing caches.
• Dependent flows: repo selector UIs, explicit refresh actions, GitHub reconnect/unlink, installation webhooks, and any future repo search consumers.
• Duplicated logic to avoid: separate search/ranking rules in UI components. Ranking should live in one server-side query path.
• Smallest coherent solution: keep the existing /api/github/installations/repos contract, but change its backend from live GitHub search to secured local search over a user-scoped cache.

Approach:

• Add a new user-scoped installation repo cache table and minimal sync metadata.
• Repurpose the GitHub repo helper into a full-sync primitive that fetches all accessible repos for one user+installation from GET /user/installations/{installationId}/repositories.
• Update /api/github/installations/repos to:
  • verify session.user.id
  • verify the installation belongs to that user
  • serve results from the local cache
  • trigger a sync when the cache is missing, explicitly refreshed, or marked stale
  • apply server-side ranking for search (exact > prefix > boundary/segment match > substring; tie-break by recent activity, then name)
• Keep cache scope strictly per user. No cross-user deduplication, no shared installation cache, no global search index.
• Remove client-side resorting so UI preserves server ranking.
• Mark cache state stale when installation repository access changes, and clear user-scoped cache rows when GitHub access is disconnected/unlinked.

Changes:

• apps/web/lib/db/schema.ts
  • Add a new table for cached installation repos keyed by userId, installationId, and repo identity.
  • Add minimal installation sync metadata needed to determine whether repo cache is fresh or stale.
• apps/web/lib/db/installations.ts or a new colocated data-access file
  • Add helpers to upsert, replace, query, and clear cached repos for one user+installation.
  • Add helpers to mark installation repo cache stale/fresh.
• apps/web/lib/github/installation-repos.ts
  • Replace early-stop search behavior with full enumeration/sync behavior.
  • Keep the fetch source user-scoped by continuing to use the user's OAuth token.
• apps/web/app/api/github/installations/repos/route.ts
  • Preserve access checks.
  • Switch reads to cached search.
  • Support sync on cache miss, stale cache, and explicit refresh.
  • Centralize search ranking and limit handling here or in a dedicated data helper.
• apps/web/app/api/github/webhook/route.ts
  • On installation / installation_repositories events, mark affected repo caches stale for users tied to that installation.
• apps/web/app/api/auth/github/unlink/route.ts and any related disconnect flow
  • Clear the current user's cached installation repos when GitHub access is removed.
• apps/web/components/repo-selector-compact.tsx
  • Stop re-sorting API results client-side.
  • Keep debounce and refresh UX, but rely on server-ranked results.
• apps/web/components/repo-selector.tsx
  • Keep parity with the compact selector so both use the same backend semantics.
• apps/web/lib/github/installation-repos.test.ts
  • Replace tests that assert early-stop paging with tests for full sync behavior.
• Add API/data tests
  • Verify user A cannot read user B's cached repos, even for the same GitHub installation/account.
  • Verify stale-cache refresh, explicit refresh, and ranking for short queries like 2-letter repo names.

Verification:

• End-to-end behavior:
  • First selector open with no cache performs one sync and then serves local search results.
  • Subsequent searches do not hit GitHub repeatedly.
  • Short queries (for example ui, db) return exact/prefix matches correctly.
  • Refresh repopulates cache and updates results.
  • Installation webhook marks cache stale and next read re-syncs.
  • Disconnect/unlink removes cached repos for that user.
• Security checks:
  • API returns 403 for installations not owned by the session user.
  • Cached rows are only queried by matching userId + installationId.
  • Tests cover two users with overlapping installation IDs/accounts and confirm isolation.
• Relevant commands after implementation:
  • bun install if dependencies are not present
  • bun run ci
  • targeted bun tests for repo-cache and GitHub API route coverage