feat(services): add memory service — model, repository, service layer

- services/memory/model: Memory struct with categories and decay
- services/memory/repository: PostgreSQL CRUD + keyword search
- services/memory/service: dedup, decay, keyword extraction
- tests coming in follow-up commit

backend/services/memory/model.go

@@ -0,0 +1,49 @@
+// Package memory implements the long-term memory system for Cyrene.
+// It provides CRUD operations, semantic search via pgvector, deduplication, and decay.
+package memory
+
+import "time"
+
+// Memory represents a stored memory entry.
+type Memory struct {
+	ID        string    `json:"id"`
+	UserID    string    `json:"user_id"`
+	SourceID  string    `json:"source_id"`  // Group/private chat source
+	Content   string    `json:"content"`
+	Summary   string    `json:"summary"`    // LLM-generated summary
+	Keywords  []string  `json:"keywords"`   // Extracted keywords
+	Category  string    `json:"category"`   // "fact", "preference", "event", "plan"
+	Priority  float64   `json:"priority"`   // 0.0 - 1.0
+	Decay     float64   `json:"decay"`      // Decay factor, starts at 1.0
+	AccessCnt int       `json:"access_count"`
+	LastAcc   time.Time `json:"last_accessed_at"`
+	CreatedAt time.Time `json:"created_at"`
+	UpdatedAt time.Time `json:"updated_at"`
+}
+
+// SearchQuery represents a memory search request.
+type SearchQuery struct {
+	UserID   string
+	SourceID string // Optional, filter by source
+	Query    string
+	Limit    int
+	Category string // Optional filter
+}
+
+// MemoryResult wraps a memory with its search relevance score.
+type MemoryResult struct {
+	Memory Memory  `json:"memory"`
+	Score  float64 `json:"score"`
+}
+
+// Category constants
+const (
+	CatFact       = "fact"
+	CatPreference = "preference"
+	CatEvent      = "event"
+	CatPlan       = "plan"
+	CatForgotten  = "forgotten"
+)
+
+// DecayThreshold is the decay value below which a memory is considered forgotten.
+const DecayThreshold = 0.1