feat: add runtime statistics to healthcheck endpoint

Add the following counters to the healthcheck JSON response:

- sessions: current active session count (from DB)
- clients: current connected client count (from DB)
- queuedLines: total entries in client output queues (from DB)
- channels: current channel count (from DB)
- connectionsSinceBoot: total client connections since server start
- sessionsSinceBoot: total sessions created since server start
- messagesSinceBoot: total PRIVMSG/NOTICE messages since server start

Implementation:
- New internal/stats package with atomic counters for boot-scoped metrics
- New DB queries GetClientCount and GetQueueEntryCount
- Healthcheck.Healthcheck() now accepts context for DB queries
- Counter increments in session creation, registration, login, and messaging
- Stats tracker wired via Uber fx dependency injection
- Unit tests for stats package (100% coverage) and integration tests
- README updated with full healthcheck response documentation

closes #74

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)