feat: add runtime statistics to healthcheck endpoint (#80)

## Summary

Expands the `/.well-known/healthcheck.json` endpoint with runtime statistics, giving operators visibility into server load and usage patterns.

closes #74

## New healthcheck fields

| Field | Source | Description |
|-------|--------|-------------|
| `sessions` | DB | Current active session count |
| `clients` | DB | Current connected client count |
| `queuedLines` | DB | Total entries in client output queues |
| `channels` | DB | Current channel count |
| `connectionsSinceBoot` | Memory | Total client connections since server start |
| `sessionsSinceBoot` | Memory | Total sessions created since server start |
| `messagesSinceBoot` | Memory | Total PRIVMSG/NOTICE messages since server start |

## Implementation

- **New `internal/stats` package** — atomic counters for boot-scoped metrics (`connectionsSinceBoot`, `sessionsSinceBoot`, `messagesSinceBoot`). Thread-safe via `sync/atomic`.
- **New DB queries** — `GetClientCount()` and `GetQueueEntryCount()` for current snapshot counts.
- **Healthcheck changes** — `Healthcheck()` now accepts `context.Context` to query the database. Response struct extended with all 7 new fields. DB-derived stats populated with graceful error handling (logged, not fatal).
- **Counter instrumentation** — Increments added at:
  - `handleCreateSession` → `IncrSessions` + `IncrConnections`
  - `handleRegister` → `IncrSessions` + `IncrConnections`
  - `handleLogin` → `IncrConnections` (new client for existing session)
  - `handlePrivmsg` → `IncrMessages` (covers both PRIVMSG and NOTICE)
- **Wired via fx** — `stats.Tracker` provided through Uber fx DI in both production and test setups.

## Tests

- `internal/stats/stats_test.go` — 5 tests covering all counter operations (100% coverage)
- `TestHealthcheckRuntimeStatsFields` — verifies all 7 new fields are present in the response
- `TestHealthcheckRuntimeStatsValues` — end-to-end: creates a session, joins a channel, sends a message, then verifies counts are nonzero

## README

Updated healthcheck documentation with full response shape, field descriptions, and project structure listing for `internal/stats/`.

Co-authored-by: user <user@Mac.lan guest wan>
Reviewed-on: #80
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

README.md

@@ -1399,13 +1399,40 @@ Return server metadata. No authentication required.
 
 ### GET /.well-known/healthcheck.json — Health Check
 
-Standard health check endpoint. No authentication required.
+Standard health check endpoint. No authentication required. Returns server
+health status and runtime statistics.
 
 **Response:** `200 OK`
+
 ```json
-{"status": "ok"}
+{
+  "status": "ok",
+  "now": "2024-01-15T12:00:00.000000000Z",
+  "uptimeSeconds": 3600,
+  "uptimeHuman": "1h0m0s",
+  "version": "0.1.0",
+  "appname": "neoirc",
+  "maintenanceMode": false,
+  "sessions": 42,
+  "clients": 85,
+  "queuedLines": 128,
+  "channels": 7,
+  "connectionsSinceBoot": 200,
+  "sessionsSinceBoot": 150,
+  "messagesSinceBoot": 5000
+}
 ```
 
+| Field                  | Description                                       |
+| ---------------------- | ------------------------------------------------- |
+| `sessions`             | Current number of active sessions                 |
+| `clients`              | Current number of connected clients               |
+| `queuedLines`          | Total entries in client output queues              |
+| `channels`             | Current number of channels                        |
+| `connectionsSinceBoot` | Total client connections since server start        |
+| `sessionsSinceBoot`    | Total sessions created since server start          |
+| `messagesSinceBoot`    | Total PRIVMSG/NOTICE messages sent since server start |
+
 ---
 
 ## Message Flow
@@ -2332,6 +2359,8 @@ neoirc/
 │   │   └── healthcheck.go  # Health check handler
 │   ├── healthcheck/        # Health check logic
 │   │   └── healthcheck.go
+│   ├── stats/              # Runtime statistics (atomic counters)
+│   │   └── stats.go
 │   ├── logger/             # slog-based logging
 │   │   └── logger.go
 │   ├── middleware/          # HTTP middleware (logging, CORS, metrics, auth)