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/database/migrations/postgres/007_workflows.sql

@@ -78,3 +78,55 @@ CREATE TABLE IF NOT EXISTS workflow_versions (
 
 CREATE INDEX IF NOT EXISTS idx_workflow_versions_workflow
     ON workflow_versions(workflow_id, version_number DESC);
+
+-- ── Workflow Instances (v0.3.1) ──────────────
+
+CREATE TABLE IF NOT EXISTS workflow_instances (
+    id                UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    workflow_id       UUID NOT NULL REFERENCES workflows(id) ON DELETE CASCADE,
+    workflow_version  INTEGER NOT NULL,
+    current_stage     TEXT NOT NULL,
+    stage_data        JSONB NOT NULL DEFAULT '{}',
+    status            TEXT NOT NULL DEFAULT 'active'
+                          CHECK (status IN ('active', 'completed', 'cancelled', 'stale', 'error')),
+    started_by        TEXT NOT NULL,
+    entry_token       TEXT,
+    metadata          JSONB NOT NULL DEFAULT '{}',
+    stage_entered_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    created_at        TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at        TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX IF NOT EXISTS idx_workflow_instances_workflow_status
+    ON workflow_instances(workflow_id, status);
+CREATE INDEX IF NOT EXISTS idx_workflow_instances_active
+    ON workflow_instances(status) WHERE status = 'active';
+CREATE UNIQUE INDEX IF NOT EXISTS idx_workflow_instances_token
+    ON workflow_instances(entry_token) WHERE entry_token IS NOT NULL;
+
+DROP TRIGGER IF EXISTS workflow_instances_updated_at ON workflow_instances;
+CREATE TRIGGER workflow_instances_updated_at BEFORE UPDATE ON workflow_instances
+    FOR EACH ROW EXECUTE FUNCTION update_updated_at();
+
+-- ── Workflow Assignments (v0.3.1) ────────────
+
+CREATE TABLE IF NOT EXISTS workflow_assignments (
+    id            UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    instance_id   UUID NOT NULL REFERENCES workflow_instances(id) ON DELETE CASCADE,
+    stage         TEXT NOT NULL,
+    team_id       UUID NOT NULL REFERENCES teams(id) ON DELETE CASCADE,
+    assigned_to   UUID REFERENCES users(id) ON DELETE SET NULL,
+    status        TEXT NOT NULL DEFAULT 'unassigned'
+                      CHECK (status IN ('unassigned', 'claimed', 'completed', 'cancelled')),
+    review_data   JSONB NOT NULL DEFAULT '{}',
+    claimed_at    TIMESTAMPTZ,
+    completed_at  TIMESTAMPTZ,
+    created_at    TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX IF NOT EXISTS idx_workflow_assignments_instance_stage
+    ON workflow_assignments(instance_id, stage);
+CREATE INDEX IF NOT EXISTS idx_workflow_assignments_team_status
+    ON workflow_assignments(team_id, status);
+CREATE INDEX IF NOT EXISTS idx_workflow_assignments_assigned
+    ON workflow_assignments(assigned_to) WHERE assigned_to IS NOT NULL;