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.

rps_tracker.go

@@ -2,7 +2,6 @@ package main
 
 import (
 	"context"
-	"sync"
 	"sync/atomic"
 	"time"
 )
@@ -10,9 +9,8 @@ import (
 // RPSTracker tracks requests per second using periodic sampling
 type RPSTracker struct {
 	lastCount      atomic.Int64
-	lastSampleTime atomic.Int64 // Unix nano
-	currentRPS     uint64       // stored as uint64, accessed with atomic operations
-	mu             sync.RWMutex // for currentRPS updates
+	lastSampleTime atomic.Int64  // Unix nano
+	currentRPS     atomic.Uint64 // centirps (RPS * 100)
 	ctx            context.Context
 	cancel         context.CancelFunc
 }
@@ -74,9 +72,7 @@ func (r *RPSTracker) sample() {
 	if elapsed > 0 {
 		rps := float64(currentCount) / elapsed
 		// Store RPS as centirps for precision (multiply by 100)
-		r.mu.Lock()
-		atomic.StoreUint64(&r.currentRPS, uint64(rps*100))
-		r.mu.Unlock()
+		r.currentRPS.Store(uint64(rps * 100))
 	}
 
 	// Reset for next sample
@@ -86,9 +82,7 @@ func (r *RPSTracker) sample() {
 
 // GetCurrentRPS returns the current requests per second
 func (r *RPSTracker) GetCurrentRPS() float64 {
-	r.mu.RLock()
-	centirps := atomic.LoadUint64(&r.currentRPS)
-	r.mu.RUnlock()
+	centirps := r.currentRPS.Load()
 	return float64(centirps) / 100.0
 }