feat: reword phase to epic, update mkdocs

docs/content/stories/epic6/6.4-rate-limiting.md

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+# Story 6.4: Rate Limiting
+
+## Metadata
+- **Story ID**: 6.4
+- **Title**: Rate Limiting
+- **Epic**: 6 - Observability & Production Readiness
+- **Status**: Pending
+- **Priority**: High
+- **Estimated Time**: 4-5 hours
+- **Dependencies**: 1.5, 5.1
+
+## Goal
+Implement rate limiting to prevent API abuse and ensure fair resource usage.
+
+## Description
+This story implements rate limiting middleware that limits requests per user and per IP address, with configurable limits per endpoint.
+
+## Deliverables
+
+### 1. Rate Limiting Middleware
+- Per-user rate limiting
+- Per-IP rate limiting
+- Configurable limits per endpoint
+- Rate limit storage (Redis)
+- Return `X-RateLimit-*` headers
+
+### 2. Configuration
+- Rate limit config in `config/default.yaml`:
+  ```yaml
+  rate_limiting:
+    enabled: true
+    per_user: 100/minute
+    per_ip: 1000/minute
+  ```
+
+### 3. Integration
+- Integrate with HTTP server
+- Add to middleware stack
+- Error responses for rate limit exceeded
+
+## Acceptance Criteria
+- [ ] Rate limiting prevents abuse
+- [ ] Per-user limits work correctly
+- [ ] Per-IP limits work correctly
+- [ ] Rate limit headers are returned
+- [ ] Configuration is flexible
+- [ ] Rate limits are stored in Redis
+
+## Files to Create/Modify
+- `internal/server/middleware.go` - Rate limiting middleware
+- `internal/infra/ratelimit/limiter.go` - Rate limiter implementation
+- `config/default.yaml` - Add rate limit config
+