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.

Makefile

@@ -1,6 +1,14 @@
 CI_RUN?=false
 TIMESTAMP := $(shell date +%Y%m%d-%H%M%S)
 
+# Build hardening flags
+# -s: omit symbol table, -w: omit DWARF debug info (smaller binaries)
+LDFLAGS ?= -s -w
+# -trimpath: remove local filesystem paths from binary (reproducible builds)
+GOFLAGS ?= -trimpath
+# CGO_ENABLED=0: static binary, no libc dependency (distroless-friendly)
+export CGO_ENABLED = 0
+
 # ADDITIONAL_BUILD_FLAGS=""
 
 # ifeq ($(CI_RUN), true)
@@ -17,15 +25,15 @@ run: build ## run application
 
 .PHONY: build
 build: ## build the binary
-	go build -o graphql-proxy *.go
+	go build $(GOFLAGS) -ldflags="$(LDFLAGS)" -o graphql-proxy *.go
 
 .PHONY: test
 test: ## run tests on library
-	@LOG_LEVEL=info go test -v -cover -race ./...
+	@CGO_ENABLED=1 LOG_LEVEL=info go test -v -cover -race ./...
 
 .PHONY: test-packages
 test-packages: ## run tests on packages
-	@go test -v -cover ./pkg/...
+	@CGO_ENABLED=1 go test -v -cover -race ./pkg/...
 
 .PHONY: all
 all: test-packages test
@@ -37,11 +45,11 @@ update: ## update dependencies
 
 .PHONY: build-amd64
 build-amd64: ## build the Linux AMD64 binary
-	GOOS=linux GOARCH=amd64 go build -o graphql-proxy-amd64 *.go
+	GOOS=linux GOARCH=amd64 go build $(GOFLAGS) -ldflags="$(LDFLAGS)" -o graphql-proxy-amd64 *.go
 
 .PHONY: build-arm64
 build-arm64: ## build the Linux ARM64 binary
-	GOOS=linux GOARCH=arm64 go build -o graphql-proxy-arm64 *.go
+	GOOS=linux GOARCH=arm64 go build $(GOFLAGS) -ldflags="$(LDFLAGS)" -o graphql-proxy-arm64 *.go
 
 .PHONY: build-all
 build-all: build-amd64 build-arm64 ## build both AMD64 and ARM64 binaries