Feat v0.3.1 instance + assignment tables and store

Add workflow execution layer: workflow_instances tracks running workflows
(stage, status, entry token), workflow_assignments tracks per-stage team
queue with optimistic-lock claim. 15 new store methods in PG + SQLite.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/models/workflow.go

@@ -432,6 +432,76 @@ func evaluateFieldCondition(cond *FieldCondition, data map[string]interface{}) b
 	}
 }
 
+// ── Instance Status Constants ───────────────
+
+const (
+	InstanceStatusActive    = "active"
+	InstanceStatusCompleted = "completed"
+	InstanceStatusCancelled = "cancelled"
+	InstanceStatusStale     = "stale"
+	InstanceStatusError     = "error"
+)
+
+// ValidInstanceStatuses is the set of valid workflow instance status values.
+var ValidInstanceStatuses = map[string]bool{
+	InstanceStatusActive:    true,
+	InstanceStatusCompleted: true,
+	InstanceStatusCancelled: true,
+	InstanceStatusStale:     true,
+	InstanceStatusError:     true,
+}
+
+// ── Assignment Status Constants ─────────────
+
+const (
+	AssignmentStatusUnassigned = "unassigned"
+	AssignmentStatusClaimed    = "claimed"
+	AssignmentStatusCompleted  = "completed"
+	AssignmentStatusCancelled  = "cancelled"
+)
+
+// ValidAssignmentStatuses is the set of valid workflow assignment status values.
+var ValidAssignmentStatuses = map[string]bool{
+	AssignmentStatusUnassigned: true,
+	AssignmentStatusClaimed:    true,
+	AssignmentStatusCompleted:  true,
+	AssignmentStatusCancelled:  true,
+}
+
+// ── Workflow Instance ───────────────────────
+
+// WorkflowInstance tracks a single execution of a workflow definition.
+type WorkflowInstance struct {
+	ID              string          `json:"id"`
+	WorkflowID      string          `json:"workflow_id"`
+	WorkflowVersion int             `json:"workflow_version"`
+	CurrentStage    string          `json:"current_stage"`
+	StageData       json.RawMessage `json:"stage_data"`
+	Status          string          `json:"status"`
+	StartedBy       string          `json:"started_by"`
+	EntryToken      *string         `json:"entry_token,omitempty"`
+	Metadata        json.RawMessage `json:"metadata"`
+	StageEnteredAt  time.Time       `json:"stage_entered_at"`
+	CreatedAt       time.Time       `json:"created_at"`
+	UpdatedAt       time.Time       `json:"updated_at"`
+}
+
+// ── Workflow Assignment ─────────────────────
+
+// WorkflowAssignment tracks per-stage work items in the team queue.
+type WorkflowAssignment struct {
+	ID          string          `json:"id"`
+	InstanceID  string          `json:"instance_id"`
+	Stage       string          `json:"stage"`
+	TeamID      string          `json:"team_id"`
+	AssignedTo  *string         `json:"assigned_to,omitempty"`
+	Status      string          `json:"status"`
+	ReviewData  json.RawMessage `json:"review_data"`
+	ClaimedAt   *time.Time      `json:"claimed_at,omitempty"`
+	CompletedAt *time.Time      `json:"completed_at,omitempty"`
+	CreatedAt   time.Time       `json:"created_at"`
+}
+
 // ── Workflow Version (immutable snapshot) ───
 
 // WorkflowVersion is an immutable snapshot of a workflow definition