docs: add mkdocs, update links, add architecture documentation

docs/content/adr/0014-health-check-implementation.md

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+# ADR-0014: Health Check Implementation
+
+## Status
+Accepted
+
+## Context
+The platform needs health check endpoints for:
+- Kubernetes liveness probes (`/healthz`)
+- Kubernetes readiness probes (`/ready`)
+- Monitoring and alerting
+- Load balancer health checks
+
+Health checks should be:
+- Fast and lightweight
+- Check critical dependencies (database, cache, etc.)
+- Provide clear status indicators
+
+## Decision
+Implement **custom health check registry** with composable checkers:
+
+1. **Liveness endpoint** (`/healthz`): Always returns 200 if process is running
+2. **Readiness endpoint** (`/ready`): Checks all registered health checkers
+3. **Health check interface**: `type HealthChecker interface { Check(ctx context.Context) error }`
+4. **Registry pattern**: Modules can register additional health checkers
+
+**Rationale:**
+- Custom implementation gives full control
+- Composable design allows modules to add checks
+- Simple interface is easy to test
+- No external dependency for basic functionality
+- Can extend with Prometheus metrics later
+
+## Consequences
+
+### Positive
+- Lightweight and fast
+- Extensible by modules
+- Easy to test
+- Clear separation of liveness vs readiness
+
+### Negative
+- Need to implement ourselves (though simple)
+- Must maintain the registry
+
+### Implementation Notes
+- Create `pkg/health/health.go` interface
+- Implement `internal/health/registry.go` with checker map
+- Register core checkers: database, cache (if enabled)
+- Add endpoints to HTTP router
+- Return JSON response: `{"status": "ok", "checks": {...}}`
+- Consider timeout (e.g., 5 seconds) for readiness checks
+