feat: scaffold app skeleton with config, ULID, health endpoints, compose stack (phase 1)

- internal .env loader (real env wins) + strictly validated config struct
- ULID package: 48-bit ms timestamp + 80-bit crypto randomness, sortable
- HTTP server with recovery/request-id/trusted-proxy/access-log middleware
- /healthz, /readyz (pluggable checkers), /metricsz (counter registry)
- multi-stage Dockerfile (alpine, non-root) + compose with healthchecks,
  app bound to 127.0.0.1:8787, mongo unpublished, graceful 15s shutdown

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

internal/metrics/metrics.go

@@ -0,0 +1,58 @@
+// Package metrics is a tiny in-process registry of named counters and gauges
+// exposed as JSON at /metricsz. No external metrics system is required.
+package metrics
+
+import (
+	"sync"
+	"sync/atomic"
+)
+
+type Registry struct {
+	mu       sync.RWMutex
+	counters map[string]*atomic.Int64
+	gauges   map[string]*atomic.Int64
+}
+
+func NewRegistry() *Registry {
+	return &Registry{
+		counters: make(map[string]*atomic.Int64),
+		gauges:   make(map[string]*atomic.Int64),
+	}
+}
+
+func (r *Registry) cell(m map[string]*atomic.Int64, name string) *atomic.Int64 {
+	r.mu.RLock()
+	c, ok := m[name]
+	r.mu.RUnlock()
+	if ok {
+		return c
+	}
+	r.mu.Lock()
+	defer r.mu.Unlock()
+	if c, ok = m[name]; !ok {
+		c = new(atomic.Int64)
+		m[name] = c
+	}
+	return c
+}
+
+// Inc adds delta to a monotonic counter.
+func (r *Registry) Inc(name string, delta int64) { r.cell(r.counters, name).Add(delta) }
+
+// Set assigns the current value of a gauge (e.g. queue depth).
+func (r *Registry) Set(name string, value int64) { r.cell(r.gauges, name).Store(value) }
+
+// Snapshot returns a point-in-time copy of all values for /metricsz.
+func (r *Registry) Snapshot() (counters, gauges map[string]int64) {
+	r.mu.RLock()
+	defer r.mu.RUnlock()
+	counters = make(map[string]int64, len(r.counters))
+	for k, v := range r.counters {
+		counters[k] = v.Load()
+	}
+	gauges = make(map[string]int64, len(r.gauges))
+	for k, v := range r.gauges {
+		gauges[k] = v.Load()
+	}
+	return counters, gauges
+}