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>

internal/handlers/api_test.go

@@ -26,6 +26,7 @@ import (
 	"git.eeqj.de/sneak/neoirc/internal/logger"
 	"git.eeqj.de/sneak/neoirc/internal/middleware"
 	"git.eeqj.de/sneak/neoirc/internal/server"
+	"git.eeqj.de/sneak/neoirc/internal/stats"
 	"go.uber.org/fx"
 	"go.uber.org/fx/fxtest"
 )
@@ -90,6 +91,7 @@ func newTestServer(
 				return cfg, nil
 			},
 			newTestDB,
+			stats.New,
 			newTestHealthcheck,
 			newTestMiddleware,
 			newTestHandlers,
@@ -144,12 +146,14 @@ func newTestHealthcheck(
 	cfg *config.Config,
 	log *logger.Logger,
 	database *db.Database,
+	tracker *stats.Tracker,
 ) (*healthcheck.Healthcheck, error) {
 	hcheck, err := healthcheck.New(lifecycle, healthcheck.Params{ //nolint:exhaustruct
 		Globals:  globs,
 		Config:   cfg,
 		Logger:   log,
 		Database: database,
+		Stats:    tracker,
 	})
 	if err != nil {
 		return nil, fmt.Errorf("test healthcheck: %w", err)
@@ -183,6 +187,7 @@ func newTestHandlers(
 	cfg *config.Config,
 	database *db.Database,
 	hcheck *healthcheck.Healthcheck,
+	tracker *stats.Tracker,
 ) (*handlers.Handlers, error) {
 	hdlr, err := handlers.New(lifecycle, handlers.Params{ //nolint:exhaustruct
 		Logger:      log,
@@ -190,6 +195,7 @@ func newTestHandlers(
 		Config:      cfg,
 		Database:    database,
 		Healthcheck: hcheck,
+		Stats:       tracker,
 	})
 	if err != nil {
 		return nil, fmt.Errorf("test handlers: %w", err)
@@ -1657,6 +1663,133 @@ func TestHealthcheck(t *testing.T) {
 	}
 }
 
+func TestHealthcheckRuntimeStatsFields(t *testing.T) {
+	tserver := newTestServer(t)
+
+	resp, err := doRequest(
+		t,
+		http.MethodGet,
+		tserver.url("/.well-known/healthcheck.json"),
+		nil,
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	defer func() { _ = resp.Body.Close() }()
+
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf(
+			"expected 200, got %d", resp.StatusCode,
+		)
+	}
+
+	var result map[string]any
+
+	decErr := json.NewDecoder(resp.Body).Decode(&result)
+	if decErr != nil {
+		t.Fatalf("decode healthcheck: %v", decErr)
+	}
+
+	requiredFields := []string{
+		"sessions", "clients", "queuedLines",
+		"channels", "connectionsSinceBoot",
+		"sessionsSinceBoot", "messagesSinceBoot",
+	}
+
+	for _, field := range requiredFields {
+		if _, ok := result[field]; !ok {
+			t.Errorf(
+				"missing field %q in healthcheck", field,
+			)
+		}
+	}
+}
+
+func TestHealthcheckRuntimeStatsValues(t *testing.T) {
+	tserver := newTestServer(t)
+
+	token := tserver.createSession("statsuser")
+
+	tserver.sendCommand(token, map[string]any{
+		commandKey: joinCmd, toKey: "#statschan",
+	})
+	tserver.sendCommand(token, map[string]any{
+		commandKey: privmsgCmd,
+		toKey:      "#statschan",
+		bodyKey:    []string{"hello stats"},
+	})
+
+	result := tserver.fetchHealthcheck(t)
+
+	assertFieldGTE(t, result, "sessions", 1)
+	assertFieldGTE(t, result, "clients", 1)
+	assertFieldGTE(t, result, "channels", 1)
+	assertFieldGTE(t, result, "queuedLines", 0)
+	assertFieldGTE(t, result, "sessionsSinceBoot", 1)
+	assertFieldGTE(t, result, "connectionsSinceBoot", 1)
+	assertFieldGTE(t, result, "messagesSinceBoot", 1)
+}
+
+func (tserver *testServer) fetchHealthcheck(
+	t *testing.T,
+) map[string]any {
+	t.Helper()
+
+	resp, err := doRequest(
+		t,
+		http.MethodGet,
+		tserver.url("/.well-known/healthcheck.json"),
+		nil,
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	defer func() { _ = resp.Body.Close() }()
+
+	if resp.StatusCode != http.StatusOK {
+		t.Fatalf(
+			"expected 200, got %d", resp.StatusCode,
+		)
+	}
+
+	var result map[string]any
+
+	decErr := json.NewDecoder(resp.Body).Decode(&result)
+	if decErr != nil {
+		t.Fatalf("decode healthcheck: %v", decErr)
+	}
+
+	return result
+}
+
+func assertFieldGTE(
+	t *testing.T,
+	result map[string]any,
+	field string,
+	minimum float64,
+) {
+	t.Helper()
+
+	val, ok := result[field].(float64)
+	if !ok {
+		t.Errorf(
+			"field %q: not a number (got %T)",
+			field, result[field],
+		)
+
+		return
+	}
+
+	if val < minimum {
+		t.Errorf(
+			"expected %s >= %v, got %v",
+			field, minimum, val,
+		)
+	}
+}
+
 func TestRegisterValid(t *testing.T) {
 	tserver := newTestServer(t)