Audit pass: cookie auth migration, route guards, GP% clamp, CSV export, OCR transaction safety, N+1 fix, and cleanup

- Migrated all 435 frontend fetch calls from Authorization Bearer header to credentials: 'include' (cookie auth)
- Removed ?token= from all file/image URLs (browser history exposure)
- Added ProtectedRoute wrapper to all capability-gated routes in App.tsx
- OCR background task: added transaction boundary, improved error handling and status rollback
- DuplicateDetector: wrapped in non-fatal try/except so crashes don't abort invoice processing
- File upload: commit DB row before writing to disk to prevent orphaned files
- GP% clamped to 100% in GPReport (credit notes can inflate above 100%)
- Added CSV export to GPReport (suppliers, daily data, allowances breakdown)
- Backend file-serving endpoints: cookie auth with ?token= fallback for backward compatibility
- DATA_DIR: moved from hardcoded /app/data to os.getenv in invoices.py and recipes.py
- N+1 fix in list_recipes: batch-loads latest cost snapshot in 1 query (was N)
- Zero-yield sub-recipe: logs warning instead of silently zeroing cost contribution
- Budget spend rate input: rejects negative values
- GPReport allowances toggle: persisted to localStorage across page loads
- DB pool_size/max_overflow: configurable via DB_POOL_SIZE/DB_MAX_OVERFLOW env vars
- Fixed SyntaxWarning from \\d in invoices.py docstring

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/AllowancesReport.tsx

@@ -255,7 +255,7 @@ export default function AllowancesReport() {
     queryKey: ['allowances-summary', submittedFromDate, submittedToDate],
     queryFn: async () => {
       const res = await fetch(`/kitchen/api/reports/allowances/summary?from_date=${submittedFromDate}&to_date=${submittedToDate}`, {
-        headers: { Authorization: `Bearer ${token}` },
+        credentials: 'include',
       })
       if (!res.ok) throw new Error('Failed to fetch allowances summary')
       return res.json()
@@ -269,7 +269,7 @@ export default function AllowancesReport() {
     queryKey: ['allowances-daily', submittedFromDate, submittedToDate],
     queryFn: async () => {
       const res = await fetch(`/kitchen/api/reports/allowances/daily?from_date=${submittedFromDate}&to_date=${submittedToDate}`, {
-        headers: { Authorization: `Bearer ${token}` },
+        credentials: 'include',
       })
       if (!res.ok) throw new Error('Failed to fetch chart data')
       return res.json()
@@ -283,7 +283,7 @@ export default function AllowancesReport() {
     queryKey: ['disputes-period-summary', submittedFromDate, submittedToDate],
     queryFn: async () => {
       const res = await fetch(`/kitchen/api/reports/disputes/period-summary?from_date=${submittedFromDate}&to_date=${submittedToDate}`, {
-        headers: { Authorization: `Bearer ${token}` },
+        credentials: 'include',
       })
       if (!res.ok) throw new Error('Failed to fetch disputes summary')
       return res.json()