feat(server): git polling sync as webhook alternative for non-admin repos

[dvcdsys]

Summary

Adds git polling as an alternative to webhooks for keeping server-cloned repos in sync. Webhooks require admin:repo_hook to install, so repos you can only clone (not administer) could never auto-sync. Polling periodically fetches and re-indexes those repos.

What changed

• Shared scheduler (internal/pollscheduler): one background goroutine drives polling for all polling repos, enqueueing clone_repo jobs into the existing bounded queue (CIX_WORKER_CONCURRENCY). No new queue, no per-repo goroutines — a fleet coming due at once queues and drains instead of stampeding the indexer.
• Reuses the clone→index pipeline verbatim: PAT-authenticated git fetch (stays within rate limits, no REST calls) + incremental tree.Diff reindex. Full reindex only in the same edge cases as webhooks.
• Cadence measured from the end of the last index run: completion handlers set next_poll_at = now + interval; the scheduler writes a provisional floor at enqueue for crash-safety.
• Webhook XOR polling: enabling polling requires webhook_mode='disabled' (422 otherwise). When webhook_mode=auto registration fails (non-admin), the server auto-falls back to disabled + polling.
• Schema: git_repos gains polling_enabled / poll_interval_seconds / next_poll_at via migration 9 (git_repos_polling), idempotent; next_poll_at exposed on the GitRepo API.
• Config: CIX_DEFAULT_POLL_INTERVAL (5m), CIX_MIN_POLL_INTERVAL (60s), CIX_POLL_SCHEDULER_TICK (30s).
• API: polling fields on AddGitRepoRequest + PATCH /api/v1/projects/{hash}/git-repo.
• Docs: new doc/POLLING.md, cross-linked from WEBHOOKS.md + CONFIG_REFERENCE.md.

Reviewer notes

• Rebased onto the Managed Tunnels work already on develop; the migration is numbered 9 (8 = tunnel_config). No version reuse.
• openapi.gen.go was regenerated from the merged spec (go generate ./internal/httpapi/openapi/), not hand-edited.

Test plan

• go build ./... and go vet ./... clean
• go test ./... green (migration idempotency, gitrepos polling methods, scheduler tick + dedupe, reschedule helper, HTTP gating/fallback/PATCH)
• Manual e2e against a real non-admin repo: enable polling, confirm next_poll_at advances from cycle end, push a commit and confirm incremental reindex, verify jobs drain at CIX_WORKER_CONCURRENCY

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