Refactor RepoDiffer PR monitoring to hexagonal ports/adapters

[ponbac]

Summary

RepoDiffer currently mixes multiple concerns in one type:

• domain diff logic
• Azure DevOps provider calls
• Tokio worker orchestration (mpsc, interval, retry)
• cache/state ownership
• notification side effects

This creates tight coupling between domain logic and provider/runtime details, and makes it harder to evolve pull request monitoring toward the same hexagonal structure used by time tracking.

Goal

Refactor PR monitoring so business logic is provider-agnostic and runtime/provider specifics are isolated in adapters.

Proposed architecture

Inbound ports (use-cases)

• PullRequestMonitorService
  • start(repo_key, interval)
  • stop(repo_key)
  • force_update(repo_key)
  • get_status(repo_key) / list_statuses()
• PullRequestReadService
  • get_cached_repo_pull_requests(repo_key)
  • get_followed_pull_requests(user_id)
  • get_most_recent_commits(repo_key)

Outbound ports

• PullRequestProvider
  • fetch open PRs + required details (threads, commits, work items, identities)
• PullRequestCache
  • read/write snapshots, identities cache, monitor status, timestamps
• PullRequestNotifier
  • publish PR change events / notify affected users

Domain services

• PullRequestPollingServiceImpl
  • orchestrates provider fetch -> diff -> cache update -> notify
• PullRequestDiffEngine
  • pure PR snapshot diffing (testable, provider-independent)

Adapter boundaries

• adapters/outbound/azure_devops/* implements PullRequestProvider
• cache adapter in adapters/outbound/{postgres|memory} implements PullRequestCache
• notification adapter wraps existing notification flow behind PullRequestNotifier
• background worker (tokio::spawn, channels, retry policy) moved to an inbound/background adapter; domain service does not own mpsc/interval loop.

Migration plan (incremental)

1. Add PR ports + domain service interfaces (no behavior change yet).
2. Extract diff logic from RepoDiffer::tick into PullRequestDiffEngine.
3. Introduce PullRequestProvider Azure adapter.
4. Introduce PullRequestCache adapter (memory first to preserve current behavior).
5. Add background worker adapter and route /differs endpoints through PullRequestMonitorService.
6. Migrate pull request read endpoints (and work-item PR enrichment usage) to PullRequestReadService.
7. Remove legacy RepoDiffer runtime maps/channels from AppState.

Acceptance criteria

• Domain PR monitoring logic no longer imports az_devops::* or Tokio channel/runtime primitives.
• RepoDiffer is removed or reduced to adapter-level code.
• Routes depend on inbound service traits rather than direct AppState internals.
• Existing behavior preserved:
  • start/stop/force differ actions
  • cached PR reads
  • change notifications
• PR monitoring is structured for additional SCM providers without modifying domain services.

Out of scope

• Full multi-provider implementation in this issue (only architecture + Azure adapter migration).