feat(projects): link quotes & contracts to a project (precise cockpit rollup)

- Migration 121 adds quotes.project_id + contracts.project_id (nullable FK,
  index) and backfills the unambiguous single-project-per-customer case.
- projectService rolls quotes/contracts up by project_id, with a
  customer-based fallback on pre-121 DBs (hasColumnCached guarded).
- quote/contract create+update accept an optional projectId; detail
  transforms surface it for editor prefill.
- POST /projects/:id/quotes and /:id/contracts assign endpoints.

backend/src/services/contractService.js

@@ -803,6 +803,10 @@ async function createContract(payload, adminId) {
       row.event_time_start = payload.eventTimeStart || null;
       row.event_time_end = payload.eventTimeEnd || null;
     }
+    // Migration 121 — optional link to a Project Overview project.
+    if (payload.projectId !== undefined && await hasColumnCached('contracts', 'project_id')) {
+      row.project_id = payload.projectId || null;
+    }
     const inserted = await trx('contracts').insert(row).returning('id');
     const contractId = typeof inserted[0] === 'object' ? inserted[0].id : inserted[0];
 
@@ -890,6 +894,10 @@ async function updateContract(id, payload, adminId) {
     for (const [api, col] of Object.entries(map)) {
       if (api in payload) updates[col] = payload[api] || null;
     }
+    // Migration 121 — optional Project Overview link.
+    if ('projectId' in payload && await hasColumnCached('contracts', 'project_id')) {
+      updates.project_id = payload.projectId || null;
+    }
     await trx('contracts').where({ id }).update(updates);
 
     // Replace inclusions only when the caller sent an explicit list.