feat(server): API relay gateway backend M0-M4

Gin + GORM + pure-Go SQLite. Users/auth (JWT), API key management with
quotas, proxy gateway with weighted channel failover and health checks,
usage/billing ledger, cross-protocol conversion (Anthropic Messages /
OpenAI Chat Completions / OpenAI Responses), and channel/model admin API.
Channels declare native API formats and auto-convert the rest.

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

server/internal/pkg/ratelimit/ratelimit.go

@@ -0,0 +1,73 @@
+package ratelimit
+
+import (
+	"sync"
+	"time"
+)
+
+// Limiter is a token-bucket rate limiter keyed by string.
+type Limiter struct {
+	mu    sync.Mutex
+	rate  float64 // tokens per second
+	burst float64
+	tokens map[string]*bucket
+}
+
+type bucket struct {
+	tokens   float64
+	lastFill time.Time
+}
+
+// New creates a limiter refilling `rate` tokens/sec with `burst` capacity.
+func New(rate float64, burst int) *Limiter {
+	return &Limiter{
+		rate:   rate,
+		burst:  float64(burst),
+		tokens: map[string]*bucket{},
+	}
+}
+
+// Allow checks whether `key` may take one token now.
+func (l *Limiter) Allow(key string) bool {
+	return l.Take(key, 1)
+}
+
+// Take checks whether `key` may take n tokens now.
+func (l *Limiter) Take(key string, n float64) bool {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+	now := time.Now()
+	b, ok := l.tokens[key]
+	if !ok {
+		b = &bucket{tokens: l.burst, lastFill: now}
+		l.tokens[key] = b
+	}
+	// Refill based on elapsed time.
+	elapsed := now.Sub(b.lastFill).Seconds()
+	b.tokens = minF(l.burst, b.tokens+elapsed*l.rate)
+	b.lastFill = now
+	if b.tokens >= n {
+		b.tokens -= n
+		return true
+	}
+	return false
+}
+
+// Sweep removes idle buckets to bound memory. Call periodically.
+func (l *Limiter) Sweep(olderThan time.Duration) {
+	l.mu.Lock()
+	defer l.mu.Unlock()
+	cutoff := time.Now().Add(-olderThan)
+	for k, b := range l.tokens {
+		if b.lastFill.Before(cutoff) {
+			delete(l.tokens, k)
+		}
+	}
+}
+
+func minF(a, b float64) float64 {
+	if a < b {
+		return a
+	}
+	return b
+}