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.

server.go

@@ -293,7 +293,7 @@ func processGraphQLRequest(c *fiber.Ctx) error {
 	}
 
 	// Handle caching
-	wasCached, err := handleCaching(c, parsedResult, extractedUserID)
+	wasCached, err := handleCaching(c, parsedResult, extractedUserID, extractedRoleName)
 	if err != nil {
 		return err
 	}
@@ -326,10 +326,7 @@ func extractUserInfo(c *fiber.Ctx) (string, string) {
 }
 
 // handleCaching manages the caching logic for GraphQL requests
-func handleCaching(c *fiber.Ctx, parsedResult *parseGraphQLQueryResult, userID string) (bool, error) {
-	// Extract user role for cache key (in addition to userID already passed)
-	_, userRole := extractUserInfo(c)
-
+func handleCaching(c *fiber.Ctx, parsedResult *parseGraphQLQueryResult, userID, userRole string) (bool, error) {
 	// Calculate query hash for cache key - now includes user context for security
 	calculatedQueryHash := libpack_cache.CalculateHash(c, userID, userRole)
 
@@ -393,11 +390,10 @@ func proxyAndCacheTheRequest(c *fiber.Ctx, queryCacheHash string, cacheTime int,
 
 // logAndMonitorRequest logs and monitors the request processing.
 func logAndMonitorRequest(c *fiber.Ctx, userID, opType, opName string, wasCached bool, duration time.Duration, startTime time.Time) {
+	// Low-cardinality labels only: user_id and op_name dropped to prevent Prometheus explosion.
 	labels := map[string]string{
 		"op_type": opType,
-		"op_name": opName,
 		"cached":  strconv.FormatBool(wasCached),
-		"user_id": userID,
 	}
 
 	if cfg.Server.AccessLog {