feat(workflows): admin CRUD + run-history + approvals-inbox API

GET/POST/PUT/PATCH/DELETE /api/admin/workflows with graph read/write (PUT
writes a fresh node/edge set under version+1 and bumps workflows.version so
in-flight runs keep their pinned version). Run-history (/:id/runs,
/runs/:runId/steps) and the pending-approval inbox (GET /approvals,
POST /approvals/:id/:action → actById) round it out. Gated by the workflows
flag + RBAC (view for reads, manage for writes); built-in flows refuse
delete; graph validated (exactly one trigger, unique keys, edges reference
known nodes). Route tests cover CRUD, validation, version bump, toggle,
inbox, and the 403 permission gate.

backend/server.js

@@ -707,6 +707,7 @@ app.use('/api/admin/contracts',  require('./src/routes/adminContracts'));
 app.use('/api/admin/projects',   require('./src/routes/adminProjects'));
 app.use('/api/admin/calendar',   require('./src/routes/adminCalendar'));
 app.use('/api/admin/deals',      require('./src/routes/adminDeals'));
+app.use('/api/admin/workflows',  require('./src/routes/adminWorkflows'));
 app.use('/api/admin/tax-report', require('./src/routes/adminTaxReport'));
 app.use('/api/admin/expenses',   require('./src/routes/adminExpenses'));
 app.use('/api/admin/ledger',     require('./src/routes/adminLedger'));