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>

tests/GmRelay.Bot.Tests/packages.lock.json

@@ -18,6 +18,12 @@
           "Microsoft.TestPlatform.TestHost": "17.14.1"
         }
       },
+      "SecurityCodeScan.VS2019": {
+        "type": "Direct",
+        "requested": "[5.6.7, )",
+        "resolved": "5.6.7",
+        "contentHash": "WIE9RJswdSc2j+rLz2gW6U+gMUjMHzY2j7C/CL8/R2olXNM/+twarfMnWqm+rZodDBvaYDApJyxM8mVYf9FGrQ=="
+      },
       "xunit": {
         "type": "Direct",
         "requested": "[2.9.3, )",
@@ -726,8 +732,8 @@
           "Aspire.Npgsql": "[13.2.2, )",
           "Dapper": "[2.1.72, )",
           "Dapper.AOT": "[1.0.48, )",
-          "GmRelay.ServiceDefaults": "[1.14.0, )",
-          "GmRelay.Shared": "[1.14.0, )",
+          "GmRelay.ServiceDefaults": "[1.15.0, )",
+          "GmRelay.Shared": "[1.15.0, )",
           "Microsoft.Extensions.Hosting": "[10.0.5, )",
           "Npgsql": "[10.0.2, )",
           "Telegram.Bot": "[22.9.5.3, )",
@@ -754,8 +760,8 @@
         "dependencies": {
           "Aspire.Npgsql": "[13.2.2, )",
           "Dapper": "[2.1.72, )",
-          "GmRelay.ServiceDefaults": "[1.14.0, )",
-          "GmRelay.Shared": "[1.14.0, )",
+          "GmRelay.ServiceDefaults": "[1.15.0, )",
+          "GmRelay.Shared": "[1.15.0, )",
           "Npgsql": "[10.0.2, )",
           "Telegram.Bot": "[22.9.6.1, )"
         }