feat: add hook system, QR code login, and layered cache

- Implement extensible hook system for content moderation with
  builtin and AI-powered moderation hooks
- Add QR code login feature with SSE for real-time status updates
  and scan/confirm/cancel workflow
- Introduce layered cache with local LRU + Redis backend for
  improved read performance
- Refactor post and comment services to use hook-based moderation
  instead of direct AI service calls
- Add local cache configuration options (size, buckets, ttl)

internal/cache/lru.go

@@ -0,0 +1,205 @@
+package cache
+
+import (
+	"container/list"
+	"encoding/json"
+	"sync"
+	"time"
+)
+
+type LRUOptions struct {
+	Size          int
+	DefaultExpiry time.Duration
+	Name          string
+}
+
+type LRU struct {
+	lock              sync.RWMutex
+	size              int
+	len               int
+	currentGeneration int64
+	evictList         *list.List
+	items             map[string]*list.Element
+	defaultExpiry     time.Duration
+	name              string
+}
+
+type lruEntry struct {
+	key        string
+	value      []byte
+	expires    time.Time
+	generation int64
+}
+
+func NewLRU(opts *LRUOptions) *LRU {
+	if opts.Size <= 0 {
+		opts.Size = 1000
+	}
+	return &LRU{
+		name:          opts.Name,
+		size:          opts.Size,
+		evictList:     list.New(),
+		items:         make(map[string]*list.Element, opts.Size),
+		defaultExpiry: opts.DefaultExpiry,
+	}
+}
+
+func (l *LRU) Set(key string, value any, ttl time.Duration) {
+	if ttl <= 0 {
+		ttl = l.defaultExpiry
+	}
+	l.set(key, value, ttl)
+}
+
+func (l *LRU) set(key string, value any, ttl time.Duration) {
+	var expires time.Time
+	if ttl > 0 {
+		expires = time.Now().Add(ttl)
+	}
+
+	buf, err := json.Marshal(value)
+	if err != nil {
+		return
+	}
+
+	l.lock.Lock()
+	defer l.lock.Unlock()
+
+	if ent, ok := l.items[key]; ok {
+		l.evictList.MoveToFront(ent)
+		e := ent.Value.(*lruEntry)
+		e.value = buf
+		e.expires = expires
+		if e.generation != l.currentGeneration {
+			e.generation = l.currentGeneration
+			l.len++
+		}
+		return
+	}
+
+	ent := &lruEntry{key, buf, expires, l.currentGeneration}
+	entry := l.evictList.PushFront(ent)
+	l.items[key] = entry
+	l.len++
+
+	if l.evictList.Len() > l.size {
+		l.removeElement(l.evictList.Back())
+	}
+}
+
+func (l *LRU) Get(key string) (any, bool) {
+	val, err := l.getItem(key)
+	if err != nil {
+		return nil, false
+	}
+	return val, true
+}
+
+func (l *LRU) getItem(key string) ([]byte, error) {
+	l.lock.Lock()
+	defer l.lock.Unlock()
+
+	ent, ok := l.items[key]
+	if !ok {
+		return nil, ErrKeyNotFound
+	}
+	e := ent.Value.(*lruEntry)
+	if e.generation != l.currentGeneration || (!e.expires.IsZero() && time.Now().After(e.expires)) {
+		l.removeElement(ent)
+		return nil, ErrKeyNotFound
+	}
+	l.evictList.MoveToFront(ent)
+	return e.value, nil
+}
+
+func (l *LRU) Delete(key string) {
+	l.lock.Lock()
+	defer l.lock.Unlock()
+
+	if ent, ok := l.items[key]; ok {
+		l.removeElement(ent)
+	}
+}
+
+func (l *LRU) DeleteByPrefix(prefix string) {
+	l.lock.Lock()
+	defer l.lock.Unlock()
+
+	for key, ent := range l.items {
+		if len(key) >= len(prefix) && key[:len(prefix)] == prefix {
+			l.removeElement(ent)
+		}
+	}
+}
+
+func (l *LRU) Clear() {
+	l.lock.Lock()
+	defer l.lock.Unlock()
+
+	l.len = 0
+	l.currentGeneration++
+}
+
+func (l *LRU) Exists(key string) bool {
+	l.lock.RLock()
+	defer l.lock.RUnlock()
+
+	ent, ok := l.items[key]
+	if !ok {
+		return false
+	}
+	e := ent.Value.(*lruEntry)
+	return e.generation == l.currentGeneration && (e.expires.IsZero() || !time.Now().After(e.expires))
+}
+
+func (l *LRU) Increment(key string) int64 {
+	return l.IncrementBy(key, 1)
+}
+
+func (l *LRU) IncrementBy(key string, value int64) int64 {
+	l.lock.Lock()
+	defer l.lock.Unlock()
+
+	var current int64
+	if ent, ok := l.items[key]; ok {
+		e := ent.Value.(*lruEntry)
+		if e.generation == l.currentGeneration && (e.expires.IsZero() || !time.Now().After(e.expires)) {
+			json.Unmarshal(e.value, &current)
+		}
+	}
+
+	current += value
+	buf, _ := json.Marshal(current)
+
+	if ent, ok := l.items[key]; ok {
+		e := ent.Value.(*lruEntry)
+		e.value = buf
+		l.evictList.MoveToFront(ent)
+	} else {
+		ent := &lruEntry{key, buf, time.Time{}, l.currentGeneration}
+		entry := l.evictList.PushFront(ent)
+		l.items[key] = entry
+		l.len++
+	}
+
+	return current
+}
+
+func (l *LRU) Len() int {
+	l.lock.RLock()
+	defer l.lock.RUnlock()
+	return l.len
+}
+
+func (l *LRU) Name() string {
+	return l.name
+}
+
+func (l *LRU) removeElement(e *list.Element) {
+	l.evictList.Remove(e)
+	kv := e.Value.(*lruEntry)
+	if kv.generation == l.currentGeneration {
+		l.len--
+	}
+	delete(l.items, kv.key)
+}