Add resource status + statistics APIs

### Summary
Introduce stable APIs for resource status and basic utilization statistics.

### Motivation
Operators and UIs need answers like:
- is resource X free?
- who locked it and for how long?
- average lock duration / queue wait time

Today this requires scraping UI tables and does not have a stable schema.

### Proposed API (Java)
- `getStatus(String resourceName)` -> state (FREE/LOCKED/RESERVED/QUEUED), owner/build, timestamps
- `getStats(String resourceName)` -> lockCount, totalLockTime, avgLockTime, avgQueueTime

### Proposed API surface (HTTP)
Expose read-only JSON (GET-only) under the existing root action namespace (exact endpoint to be decided), returning a documented schema.

### Implementation notes (v1)
Start with in-memory counters updated on lock/unlock/queue transitions. Persistence can be a follow-up.

### Acceptance criteria
- Status and stats change correctly across a lock/unlock cycle
- GET-only endpoint with appropriate permission checks
- Minimal overhead on hot paths

### Tests
- JenkinsRule tests proving status/stats correctness
- Endpoint test verifying verb/permissions

Start by tracing the existing lock, unlock, and queue-transition paths and the root action namespace used for HTTP endpoints. Review the proposed status and statistics fields, then use JenkinsRule tests to verify a lock/unlock cycle and an endpoint test for GET-only behavior and permissions. Done means stable documented JSON responses with correct counters and minimal hot-path overhead.

Written by the indexing model from the issue text.