infra: add PostgreSQL daily backup via pg_dump with rotation

- Add db-backup service to compose.yaml (postgres:17-alpine + cron)
- Add pgbackups volume for backup storage
- Add scripts/restore.sh for manual restore from latest backup
- Update .env.example with BACKUP_RETENTION_DAYS and BACKUP_VOLUME_NAME
- Document backup/restore flow in README

Bump version -> 1.15.0

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/GmRelay.Web/Components/Layout/NavMenu.razor

@@ -56,7 +56,7 @@
                     </button>
                 </form>
 
-                <div class="nav-version">v1.14.0</div>
+                <div class="nav-version">v1.15.0</div>
             </div>
         </Authorized>
         <NotAuthorized>

src/GmRelay.Web/packages.lock.json

@@ -32,6 +32,12 @@
         "resolved": "10.0.2",
         "contentHash": "q5RfBI+wywJSFUNDE1L4ZbHEHCFTblo8Uf6A6oe4feOUFYiUQXyAf9GBh5qEZpvJaHiEbpBPkQumjEhXCJxdrg=="
       },
+      "SecurityCodeScan.VS2019": {
+        "type": "Direct",
+        "requested": "[5.6.7, )",
+        "resolved": "5.6.7",
+        "contentHash": "WIE9RJswdSc2j+rLz2gW6U+gMUjMHzY2j7C/CL8/R2olXNM/+twarfMnWqm+rZodDBvaYDApJyxM8mVYf9FGrQ=="
+      },
       "Telegram.Bot": {
         "type": "Direct",
         "requested": "[22.9.6.1, )",