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/handlers/handlers.go

@@ -16,6 +16,7 @@ import (
 	"git.eeqj.de/sneak/neoirc/internal/hashcash"
 	"git.eeqj.de/sneak/neoirc/internal/healthcheck"
 	"git.eeqj.de/sneak/neoirc/internal/logger"
+	"git.eeqj.de/sneak/neoirc/internal/stats"
 	"go.uber.org/fx"
 )
 
@@ -30,6 +31,7 @@ type Params struct {
 	Config      *config.Config
 	Database    *db.Database
 	Healthcheck *healthcheck.Healthcheck
+	Stats       *stats.Tracker
 }
 
 const defaultIdleTimeout = 30 * 24 * time.Hour
@@ -41,6 +43,7 @@ type Handlers struct {
 	hc            *healthcheck.Healthcheck
 	broker        *broker.Broker
 	hashcashVal   *hashcash.Validator
+	stats         *stats.Tracker
 	cancelCleanup context.CancelFunc
 }
 
@@ -60,6 +63,7 @@ func New(
 		hc:          params.Healthcheck,
 		broker:      broker.New(),
 		hashcashVal: hashcash.NewValidator(resource),
+		stats:       params.Stats,
 	}
 
 	lifecycle.Append(fx.Hook{