feat(skills): add Java Spring Boot skills

Adds 6 new skills for Java Spring Boot development:
- java-coding-standards: naming, immutability, Optional, streams
- springboot-patterns: REST API, service layer, caching, async
- springboot-tdd: JUnit 5, Mockito, MockMvc, Testcontainers
- springboot-security: Spring Security, validation, CSRF
- springboot-verification: Maven/Gradle build verification
- jpa-patterns: entity design, relationships

Thanks @examin!

skills/java-coding-standards/SKILL.md

@@ -0,0 +1,138 @@
+---
+name: java-coding-standards
+description: Java coding standards for Spring Boot services: naming, immutability, Optional usage, streams, exceptions, generics, and project layout.
+---
+
+# Java Coding Standards
+
+Standards for readable, maintainable Java (17+) code in Spring Boot services.
+
+## Core Principles
+
+- Prefer clarity over cleverness
+- Immutable by default; minimize shared mutable state
+- Fail fast with meaningful exceptions
+- Consistent naming and package structure
+
+## Naming
+
+```java
+// ✅ Classes/Records: PascalCase
+public class MarketService {}
+public record Money(BigDecimal amount, Currency currency) {}
+
+// ✅ Methods/fields: camelCase
+private final MarketRepository marketRepository;
+public Market findBySlug(String slug) {}
+
+// ✅ Constants: UPPER_SNAKE_CASE
+private static final int MAX_PAGE_SIZE = 100;
+```
+
+## Immutability
+
+```java
+// ✅ Favor records and final fields
+public record MarketDto(Long id, String name, MarketStatus status) {}
+
+public class Market {
+  private final Long id;
+  private final String name;
+  // getters only, no setters
+}
+```
+
+## Optional Usage
+
+```java
+// ✅ Return Optional from find* methods
+Optional<Market> market = marketRepository.findBySlug(slug);
+
+// ✅ Map/flatMap instead of get()
+return market
+    .map(MarketResponse::from)
+    .orElseThrow(() -> new EntityNotFoundException("Market not found"));
+```
+
+## Streams Best Practices
+
+```java
+// ✅ Use streams for transformations, keep pipelines short
+List<String> names = markets.stream()
+    .map(Market::name)
+    .filter(Objects::nonNull)
+    .toList();
+
+// ❌ Avoid complex nested streams; prefer loops for clarity
+```
+
+## Exceptions
+
+- Use unchecked exceptions for domain errors; wrap technical exceptions with context
+- Create domain-specific exceptions (e.g., `MarketNotFoundException`)
+- Avoid broad `catch (Exception ex)` unless rethrowing/logging centrally
+
+```java
+throw new MarketNotFoundException(slug);
+```
+
+## Generics and Type Safety
+
+- Avoid raw types; declare generic parameters
+- Prefer bounded generics for reusable utilities
+
+```java
+public <T extends Identifiable> Map<Long, T> indexById(Collection<T> items) { ... }
+```
+
+## Project Structure (Maven/Gradle)
+
+```
+src/main/java/com/example/app/
+  config/
+  controller/
+  service/
+  repository/
+  domain/
+  dto/
+  util/
+src/main/resources/
+  application.yml
+src/test/java/... (mirrors main)
+```
+
+## Formatting and Style
+
+- Use 2 or 4 spaces consistently (project standard)
+- One public top-level type per file
+- Keep methods short and focused; extract helpers
+- Order members: constants, fields, constructors, public methods, protected, private
+
+## Code Smells to Avoid
+
+- Long parameter lists → use DTO/builders
+- Deep nesting → early returns
+- Magic numbers → named constants
+- Static mutable state → prefer dependency injection
+- Silent catch blocks → log and act or rethrow
+
+## Logging
+
+```java
+private static final Logger log = LoggerFactory.getLogger(MarketService.class);
+log.info("fetch_market slug={}", slug);
+log.error("failed_fetch_market slug={}", slug, ex);
+```
+
+## Null Handling
+
+- Accept `@Nullable` only when unavoidable; otherwise use `@NonNull`
+- Use Bean Validation (`@NotNull`, `@NotBlank`) on inputs
+
+## Testing Expectations
+
+- JUnit 5 + AssertJ for fluent assertions
+- Mockito for mocking; avoid partial mocks where possible
+- Favor deterministic tests; no hidden sleeps
+
+**Remember**: Keep code intentional, typed, and observable. Optimize for maintainability over micro-optimizations unless proven necessary.