feat(public): version/health surfacing for the app first-run probe

Expose a small backend identity/version descriptor (§8.4 of the Android plan)
so a client can positively recognize a Runic Gateway backend on first-run and
run a version-mismatch guard, instead of inferring from an incidental shape.

- New config/version.js: { service: 'runic-gateway', api: 'v1', server: <pkg> }.
- GET /public/status now includes a `version` block (the app already calls this
  on first-run, so it gets identity + version in one round trip).
- New GET /public/version: a lightweight, DB-free identity endpoint — the
  canonical target for the version guard and a cheap liveness check.
- Swagger: PublicVersion schema + version on PublicStatus; /version annotated.
- test/publicVersion.test.js covers the config shape and the DB-free 200.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NgyHnrNa8WwG3doxvxjuCr

server/swagger/swagger.js

@@ -575,6 +575,16 @@ const doc = {
         properties: {
           mode: { type: 'string', enum: ['live', 'maintenance'], example: 'live' },
           status_message: { type: 'string', example: '' },
+          version: { $ref: '#/components/schemas/PublicVersion' },
+        },
+      },
+      PublicVersion: {
+        type: 'object',
+        description: 'Backend identity + version (GET /public/version; also embedded in /public/status).',
+        properties: {
+          service: { type: 'string', example: 'runic-gateway', description: 'Stable backend identifier for first-run recognition.' },
+          api: { type: 'string', example: 'v1', description: 'API contract version (matches the /api/v1 mount).' },
+          server: { type: 'string', example: '1.0.0', description: 'Server package version (informational).' },
         },
       },
       // Delete/mutation acknowledgements — each echoes the affected resource key