T-2: add multi-stage Dockerfile and SPA-serving Spring Boot config

Replace server.servlet.context-path=/api with addPathPrefix so API
endpoints stay under /api while static resources and SPA routes are
served at /. Spring Boot falls back to index.html for unknown paths
(SPA forwarding). Multi-stage Dockerfile builds frontend (Node 24)
and backend (Temurin 25) into a single 250MB JRE-alpine image with
Docker-native HEALTHCHECK.

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

.dockerignore

@@ -0,0 +1,38 @@
+# Build artifacts
+**/target/
+**/dist/
+**/build/
+
+# Dependencies (rebuilt inside Docker)
+**/node_modules/
+
+# IDE
+.idea/
+.vscode/
+**/*.iml
+
+# Git
+.git/
+.gitignore
+
+# CI/CD
+.gitea/
+
+# Agent/dev files
+.claude/
+.ralph/
+.rodney/
+.agent-tests/
+docs/
+spec/
+
+# OS files
+.DS_Store
+Thumbs.db
+
+# Environment
+.env
+.env.*
+
+# Generated files (rebuilt in Docker)
+frontend/src/api/schema.d.ts