Add AI agent decision memory layer with pgvector similarity search

- models/agentdecision.go: AgentDecision GORM model (context/decision as jsonb, reasoning as text)
- migrations/migrate.go: AutoMigrate AgentDecision then ALTER TABLE to add vector(1536) column and CREATE ivfflat index via raw SQL
- controllers/agentDecisionController.go: CreateAgentDecision, FindSimilarDecisions (cosine distance), UpdateDecisionOutcome
- routes/routes.go: three routes under /api/v1/internal (InternalKeyAuth applied at group level)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

models/agentdecision.go

@@ -0,0 +1,15 @@
+package models
+
+import "time"
+
+type AgentDecision struct {
+	ID                uint64     `gorm:"primaryKey"`
+	DecisionType      string     `gorm:"size:50;index"`
+	BookingID         *uint64    `gorm:"index"`
+	Context           string     `gorm:"type:jsonb"`
+	Decision          string     `gorm:"type:jsonb"`
+	Reasoning         string     `gorm:"type:text"`
+	Outcome           *string    `gorm:"size:30;index"`
+	OutcomeRecordedAt *time.Time
+	CreatedAt         time.Time
+}