Add contract e-signature step with PDF viewer and canvas pad (issue #6)

Frontend (step 5):
- PDF.js 3.11.174 from CDN renders the contract inline in a scrollable viewer
- Next button logic stays unlocked; signature pad is visually locked (greyed +
  pointer-events:none) until user scrolls to the bottom of the contract
- HTML5 canvas signature pad with mouse and touch support; Clear button resets
- Typed full name field required to confirm identity
- Fallback agreement text renders if /api/contracts/template returns 404,
  auto-marks as scrolled — photographer adds PDF later with no code change

Backend:
- GET /api/contracts/template — serves api/contracts/model-release.pdf
- POST /api/bookings/:id/sign — stamps signature image + name/date/booking ID
  onto the PDF via pdf-lib, saves to api/signed-contracts/<id>.pdf, updates
  contract_signed_at and contract_pdf_path in the bookings table, fires n8n
  webhook; gracefully skips PDF stamping if template is missing
- submitBooking() now calls POST /api/bookings then POST /api/bookings/:id/sign
  sequentially before showing the success screen

Closes #6

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

api/package-lock.json

@@ -11,7 +11,26 @@
         "better-sqlite3": "^9.4.3",
         "cors": "^2.8.5",
         "dotenv": "^16.4.5",
-        "express": "^4.18.3"
+        "express": "^4.18.3",
+        "pdf-lib": "^1.17.1"
+      }
+    },
+    "node_modules/@pdf-lib/standard-fonts": {
+      "version": "1.0.0",
+      "resolved": "https://registry.npmjs.org/@pdf-lib/standard-fonts/-/standard-fonts-1.0.0.tgz",
+      "integrity": "sha512-hU30BK9IUN/su0Mn9VdlVKsWBS6GyhVfqjwl1FjZN4TxP6cCw0jP2w7V3Hf5uX7M0AZJ16vey9yE0ny7Sa59ZA==",
+      "license": "MIT",
+      "dependencies": {
+        "pako": "^1.0.6"
+      }
+    },
+    "node_modules/@pdf-lib/upng": {
+      "version": "1.0.1",
+      "resolved": "https://registry.npmjs.org/@pdf-lib/upng/-/upng-1.0.1.tgz",
+      "integrity": "sha512-dQK2FUMQtowVP00mtIksrlZhdFXQZPC+taih1q4CvPZ5vqdxR/LKBaFg0oAfzd1GlHZXXSPdQfzQnt+ViGvEIQ==",
+      "license": "MIT",
+      "dependencies": {
+        "pako": "^1.0.10"
       }
     },
     "node_modules/accepts": {
@@ -820,6 +839,12 @@
         "wrappy": "1"
       }
     },
+    "node_modules/pako": {
+      "version": "1.0.11",
+      "resolved": "https://registry.npmjs.org/pako/-/pako-1.0.11.tgz",
+      "integrity": "sha512-4hLB8Py4zZce5s4yd9XzopqwVv/yGNhV1Bl8NTmCq1763HeK2+EwVTv+leGeL13Dnh2wfbqowVPXCIO0z4taYw==",
+      "license": "(MIT AND Zlib)"
+    },
     "node_modules/parseurl": {
       "version": "1.3.3",
       "resolved": "https://registry.npmjs.org/parseurl/-/parseurl-1.3.3.tgz",
@@ -835,6 +860,18 @@
       "integrity": "sha512-A/AGNMFN3c8bOlvV9RreMdrv7jsmF9XIfDeCd87+I8RNg6s78BhJxMu69NEMHBSJFxKidViTEdruRwEk/WIKqA==",
       "license": "MIT"
     },
+    "node_modules/pdf-lib": {
+      "version": "1.17.1",
+      "resolved": "https://registry.npmjs.org/pdf-lib/-/pdf-lib-1.17.1.tgz",
+      "integrity": "sha512-V/mpyJAoTsN4cnP31vc0wfNA1+p20evqqnap0KLoRUN0Yk/p3wN52DOEsL4oBFcLdb76hlpKPtzJIgo67j/XLw==",
+      "license": "MIT",
+      "dependencies": {
+        "@pdf-lib/standard-fonts": "^1.0.0",
+        "@pdf-lib/upng": "^1.0.1",
+        "pako": "^1.0.11",
+        "tslib": "^1.11.1"
+      }
+    },
     "node_modules/prebuild-install": {
       "version": "7.1.3",
       "resolved": "https://registry.npmjs.org/prebuild-install/-/prebuild-install-7.1.3.tgz",
@@ -1224,6 +1261,12 @@
         "node": ">=0.6"
       }
     },
+    "node_modules/tslib": {
+      "version": "1.14.1",
+      "resolved": "https://registry.npmjs.org/tslib/-/tslib-1.14.1.tgz",
+      "integrity": "sha512-Xni35NKzjgMrwevysHTCArtLDpPvye8zV/0E4EyYn43P7/7qvQwPh9BGkHewbMulVntbigmcT7rdX3BNo9wRJg==",
+      "license": "0BSD"
+    },
     "node_modules/tunnel-agent": {
       "version": "0.6.0",
       "resolved": "https://registry.npmjs.org/tunnel-agent/-/tunnel-agent-0.6.0.tgz",