Add security improvements: CSP config, body size limits, response caching, Dockerfile hardening

[AbelOsaretin]

Closes #191, Closes #193, Closes #195, Closes #197

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Refactoring (no functional or behavioral changes)
• Performance improvement
• Documentation update
• Build / CI configuration change
• Dependency update
• Other:

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Summary

This PR addresses 4 security and reliability issues:

1. Add security headers review and CSP for the API #191: Documents helmet security headers and adds CSP_DIRECTIVES env var for Content Security Policy configuration
2. Add request body size limits #193: Adds JSON_BODY_LIMIT env var with configurable request body size limit (returns 413 on oversized payloads)
3. Add response caching for public event listings #195: Adds ResponseCacheInterceptor for public event endpoints with configurable TTL cache and automatic invalidation on writes
4. Add Dockerfile hardening: non-root user and multi-stage build #197: Hardens Dockerfile with non-root user, multi-stage build, and HEALTHCHECK

Motivation / Context

These changes improve the security posture and reliability of the backend API:

• Security headers prevent common web vulnerabilities
• Body size limits prevent DoS attacks via oversized payloads
• Response caching reduces database load for read-heavy endpoints
• Dockerfile hardening follows production best practices

Closes #191, Closes #193, Closes #195, Closes #197

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