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.

tracing/tracing.go

@@ -25,6 +25,14 @@ type TracingSetup struct {
 	tracer         trace.Tracer
 }
 
+// constSpanAttrs holds attributes that are identical for every span created
+// by this package. Building the slice once at package init avoids two
+// allocations per StartSpan / StartSpanWithAttributes call.
+var constSpanAttrs = []attribute.KeyValue{
+	semconv.ServiceName("graphql-monitoring-proxy"),
+	semconv.ServiceVersion("1.0"),
+}
+
 type TraceSpanInfo struct {
 	TraceParent string `json:"traceparent"`
 }
@@ -158,12 +166,11 @@ func (ts *TracingSetup) StartSpanWithAttributes(ctx context.Context, name string
 		return trace.SpanFromContext(ctx), ctx
 	}
 
-	// Convert string attributes to KeyValue pairs
-	attributes := make([]attribute.KeyValue, 0, len(attrs)+2)
-	attributes = append(attributes,
-		semconv.ServiceName("graphql-monitoring-proxy"),
-		semconv.ServiceVersion("1.0"),
-	)
+	// Convert string attributes to KeyValue pairs.
+	// Pre-size with constants + per-call attrs, copy constant block in one shot,
+	// then append the dynamic attributes.
+	attributes := make([]attribute.KeyValue, len(constSpanAttrs), len(constSpanAttrs)+len(attrs))
+	copy(attributes, constSpanAttrs)
 
 	for k, v := range attrs {
 		attributes = append(attributes, attribute.String(k, v))