perf+coverage: optimisation pass + coverage push to ≥70%

Performance / resource usage:
- circuit_breaker_metrics: fix data race on failCounters map (RWMutex + double-checked locking)
- server.go: drop user_id and op_name metric labels (Prometheus cardinality bound); de-duplicate extractUserInfo
- graphql.go: gate runtime.ReadMemStats per-request behind ENABLE_ALLOCATION_TRACKING flag (default off)
- graphql.go: collapse two-pass AST scan into single pass; lower-case once
- sanitization.go: cache compiled redaction regexes per pattern via sync.Map; hoist inner constants to pkg vars
- proxy.go: hoist connection/timeout substrings to pkg vars; sentinel errors for static error paths; drop dead Headers map alloc
- metrics_aggregator.go: log-field allocation guarded by Logger.IsLevelEnabled
- logging/logger.go: add IsLevelEnabled helper
- lru_cache.go: 16-shard sharding, FNV-1a routing (concurrent throughput +22%)
- cache/memory/lru_memory_cache.go: gzip compress/decompress moved outside mu.Lock
- rps_tracker.go: RWMutex+uint64 -> atomic.Uint64
- retry_budget.go: drop unused mutex
- api.go: bannedUsersIDs map+RWMutex -> sync.Map (+ snapshot/replace helpers)
- tracing/tracing.go: pkg-level constSpanAttrs, copy-then-append in StartSpanWithAttributes
- admin_dashboard.go: handleStatsWebSocket reuses bytes.Buffer + json.Encoder per connection

Build / runtime:
- Makefile: -ldflags="-s -w" -trimpath, CGO_ENABLED=0 for build (=1 for test recipes)
- Dockerfile + Dockerfile.goreleaser: ENV GOMEMLIMIT=512MiB
- main.go: blank import go.uber.org/automaxprocs (cgroup-aware GOMAXPROCS)
- main.go: PPROF_PORT env var wires net/http/pprof on 127.0.0.1 only with full server timeouts
- README.md: env-var docs + metric-label docs updated; cardinality note

Test coverage push (per package):
- main 51.2% -> 74.7%
- cache 66.3% -> 93.7%
- cache/redis 45.5% -> 98.2%
- tracing 66.7% -> 72.9%
- (cache/memory 91.6%, logging 91.9%, monitoring 77.6%, pkg/pools 100% unchanged)

New test files: coverage_micro_test, coverage_extras_test, server_handlers_test,
api_health_test, admin_dashboard_cluster_test, metrics_aggregator_test, concerns_test,
cache/cache_coverage_test, cache/redis/redis_coverage_test, tracing/tracing_coverage_test.

Bug fix: connection_resilience_test.go TestIntegratedHealthManagement.health_manager_startup
was sync.Once-coupled to InitializeBackendHealth and panicked when another test (e.g. via
parseConfig) had already triggered Once. Use NewBackendHealthManager directly.

api.go

@@ -17,10 +17,7 @@ import (
 	"github.com/sony/gobreaker"
 )
 
-var (
-	bannedUsersIDs      = make(map[string]string)
-	bannedUsersIDsMutex sync.RWMutex
-)
+var bannedUsersIDs sync.Map // key: userID string, value: reason string
 
 // authMiddleware provides API key authentication for admin endpoints
 func authMiddleware(c *fiber.Ctx) error {
@@ -132,16 +129,14 @@ func periodicallyReloadBannedUsers(ctx context.Context) {
 			loadBannedUsers()
 			cfg.Logger.Debug(&libpack_logger.LogMessage{
 				Message: "Banned users reloaded",
-				Pairs:   map[string]any{"users": bannedUsersIDs},
+				Pairs:   map[string]any{"users": snapshotBannedUsers()},
 			})
 		}
 	}
 }
 
 func checkIfUserIsBanned(c *fiber.Ctx, userID string) bool {
-	bannedUsersIDsMutex.RLock()
-	_, found := bannedUsersIDs[userID]
-	bannedUsersIDsMutex.RUnlock()
+	_, found := bannedUsersIDs.Load(userID)
 
 	cfg.Logger.Debug(&libpack_logger.LogMessage{
 		Message: "Checking if user is banned",
@@ -251,9 +246,7 @@ func apiBanUser(c *fiber.Ctx) error {
 		return c.Status(fiber.StatusBadRequest).SendString("user_id and reason are required")
 	}
 
-	bannedUsersIDsMutex.Lock()
-	bannedUsersIDs[req.UserID] = req.Reason
-	bannedUsersIDsMutex.Unlock()
+	bannedUsersIDs.Store(req.UserID, req.Reason)
 
 	cfg.Logger.Info(&libpack_logger.LogMessage{
 		Message: "Banned user",
@@ -281,9 +274,7 @@ func apiUnbanUser(c *fiber.Ctx) error {
 		return c.Status(fiber.StatusBadRequest).SendString("user_id is required")
 	}
 
-	bannedUsersIDsMutex.Lock()
-	delete(bannedUsersIDs, req.UserID)
-	bannedUsersIDsMutex.Unlock()
+	bannedUsersIDs.Delete(req.UserID)
 
 	cfg.Logger.Info(&libpack_logger.LogMessage{
 		Message: "Unbanned user",
@@ -311,9 +302,7 @@ func storeBannedUsers() error {
 		}
 	}()
 
-	bannedUsersIDsMutex.RLock()
-	data, err := json.Marshal(bannedUsersIDs)
-	bannedUsersIDsMutex.RUnlock()
+	data, err := json.Marshal(snapshotBannedUsers())
 
 	if err != nil {
 		cfg.Logger.Error(&libpack_logger.LogMessage{
@@ -384,9 +373,33 @@ func loadBannedUsers() {
 		return
 	}
 
-	bannedUsersIDsMutex.Lock()
-	bannedUsersIDs = newBannedUsers
-	bannedUsersIDsMutex.Unlock()
+	replaceBannedUsers(newBannedUsers)
+}
+
+// snapshotBannedUsers returns a plain map copy of the current banned users.
+func snapshotBannedUsers() map[string]string {
+	out := make(map[string]string)
+	bannedUsersIDs.Range(func(k, v any) bool {
+		ks, kok := k.(string)
+		vs, vok := v.(string)
+		if kok && vok {
+			out[ks] = vs
+		}
+		return true
+	})
+	return out
+}
+
+// replaceBannedUsers swaps the banned users set with the provided map.
+// Existing entries are removed before inserting the new ones.
+func replaceBannedUsers(newUsers map[string]string) {
+	bannedUsersIDs.Range(func(k, _ any) bool {
+		bannedUsersIDs.Delete(k)
+		return true
+	})
+	for k, v := range newUsers {
+		bannedUsersIDs.Store(k, v)
+	}
 }
 
 func lockFile(fileLock *flock.Flock) error {