feat: client enrollment API and compute entity provisioning

Phase 1 implementation from the client-lifecycle plan.

- Migration 012: provisioning_steps table, context_version, context_files,
  enrolled_at column, slug+type index for machine entities
- API endpoints (openapi.yaml + generated code):
  POST /clients/enroll — age key issuance, Infisical identity, state transition
  GET /clients/{slug}/context — agent file delta polling (replaces git pull)
  GET /clients/{slug}/secrets — scoped secret listing
  POST /entities/provision — compute entity creation with constraint validation
  GET /entities/{slug}/provision/status — step-by-step provisioning progress
- Handlers in impl.go: enrollment with state validation and age key generation,
  provisioning with execution tracking and relationship creation,
  context endpoint with since-based delta queries
- Server struct extended with secretsBackend interface for key storage
- All tests pass, build clean

internal/db/sqlcgen/models.go

@@ -104,6 +104,18 @@ type Classification struct {
 	CreatedAt         time.Time
 }
 
+type ContextFile struct {
+	Path        string
+	Hash        string
+	LastChanged time.Time
+}
+
+type ContextVersion struct {
+	Singleton bool
+	Version   int64
+	UpdatedAt time.Time
+}
+
 type Entity struct {
 	ID               uuid.UUID
 	Slug             string
@@ -115,6 +127,8 @@ type Entity struct {
 	Version          int32
 	CreatedAt        time.Time
 	UpdatedAt        time.Time
+	EnrolledAt       *time.Time
+	EnrolledBy       *uuid.UUID
 }
 
 type EntityStatus struct {
@@ -192,6 +206,18 @@ type IdempotencyKey struct {
 	CreatedAt    time.Time
 }
 
+type KnowledgeEntity struct {
+	EntityID    uuid.UUID
+	Title       string
+	Content     string
+	Source      *string
+	Tags        []string
+	CreatedAt   time.Time
+	UpdatedAt   time.Time
+	ContentHash *string
+	Search      interface{}
+}
+
 type Ledger struct {
 	Ts             time.Time
 	ExecutionID    uuid.UUID
@@ -261,6 +287,20 @@ type Pattern struct {
 	CreatedAt       time.Time
 }
 
+type ProvisioningStep struct {
+	ID           uuid.UUID
+	EntityID     uuid.UUID
+	ExecutionID  uuid.UUID
+	StepOrder    int32
+	StepName     string
+	Status       string
+	StartedAt    *time.Time
+	FinishedAt   *time.Time
+	ErrorMessage *string
+	CreatedAt    time.Time
+	UpdatedAt    time.Time
+}
+
 type Relationship struct {
 	SourceID   uuid.UUID
 	TargetID   uuid.UUID