T-4: add JPA, Liquibase, Testcontainers, and deployment docs

Set up development infrastructure for TDD: JPA + Liquibase for
database migrations, Testcontainers for integration tests against
real PostgreSQL, profile-based configuration (prod/local), and
README deployment documentation with docker-compose example.

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

.gitignore

@@ -49,6 +49,9 @@ npm-debug.log*
 .env.*
 !.env.example
 
+# Spring Boot local profile (developer-specific, not committed)
+backend/src/main/resources/application-local.properties
+
 # Editor swap files
 *.swp
 *.swo