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

internal/db/queries.go

@@ -1266,3 +1266,42 @@ func (database *Database) PruneOldMessages(
 
 	return deleted, nil
 }
+
+// GetClientCount returns the total number of clients.
+func (database *Database) GetClientCount(
+	ctx context.Context,
+) (int64, error) {
+	var count int64
+
+	err := database.conn.QueryRowContext(
+		ctx,
+		"SELECT COUNT(*) FROM clients",
+	).Scan(&count)
+	if err != nil {
+		return 0, fmt.Errorf(
+			"get client count: %w", err,
+		)
+	}
+
+	return count, nil
+}
+
+// GetQueueEntryCount returns the total number of entries
+// in the client output queues.
+func (database *Database) GetQueueEntryCount(
+	ctx context.Context,
+) (int64, error) {
+	var count int64
+
+	err := database.conn.QueryRowContext(
+		ctx,
+		"SELECT COUNT(*) FROM client_queues",
+	).Scan(&count)
+	if err != nil {
+		return 0, fmt.Errorf(
+			"get queue entry count: %w", err,
+		)
+	}
+
+	return count, nil
+}