Add Redis Sentinel storage documentation for auth server

[tgrunnagle]

Closes #3790

Summary

Adds comprehensive documentation for configuring Redis Sentinel as the storage backend for the embedded authorization server. This completes Phase 3 (Documentation) of the Redis storage implementation plan, covering a user-facing configuration guide, architecture documentation, and example Kubernetes manifests.

Changes Made

User-Facing Guide (docs/redis-storage.md)

• Complete Redis Sentinel configuration guide with CRD and RunConfig examples
• Step-by-step Spotahome Redis Operator deployment instructions
• Redis ACL user setup with minimum required commands
• Data model reference: key schema, secondary indexes, TTL values
• Atomicity and consistency model documentation
• Troubleshooting section for connection, authentication, permission, and failover issues
• Full CRD configuration reference tables

Architecture Documentation (docs/arch/11-auth-server-storage.md)

• Storage interface design covering fosite OAuth 2.0 interfaces and ToolHive extensions
• Memory vs Redis Sentinel backend comparison with architecture diagram
• Multi-tenancy via Redis hash tags with forward-compatibility rationale
• Consistency model: Lua scripts, pipelines, and best-effort individual commands
• TTL management, serialization, and security considerations

Example Manifests (examples/operator/redis-storage/)

• redis-failover.yaml — Spotahome RedisFailover CRD with ACL file support and provisioning guidance
• sentinel-service.yaml — Manual Sentinel Service + StatefulSet with init container for writable config
• redis-credentials.yaml — Kubernetes Secret template for Redis ACL credentials
• mcpexternalauthconfig-redis-storage.yaml — Complete MCPExternalAuthConfig with Redis storage and Sentinel Service discovery

Existing Architecture Docs

• Updated docs/arch/README.md with new entry (refactor: Separate interfaces from implementation for container runtime #12), architecture map node, and Last Updated date
• Updated docs/arch/00-overview.md Related Documentation section
• Updated docs/arch/09-operator-architecture.md MCPExternalAuthConfig section with Redis storage reference

Additional Notes

• Documentation-only PR — no Go code changes
• All YAML examples validated against CRD type definitions in mcpexternalauthconfig_types.go

Large PR Justification

• Documentation updates only, all related to Authz server redis token storage

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.92%. Comparing base (1b87777) to head (70f574c).
⚠️ Report is 17 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3824      +/-   ##
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- Coverage   68.03%   67.92%   -0.11%     
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  Files         438      438              
  Lines       44000    44033      +33     
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- Hits        29936    29911      -25     
- Misses      11722    11779      +57     
- Partials     2342     2343       +1     

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[jhrozek]

Documentation review — 4 inline suggestions below. Overall the docs are thorough, accurate, and well cross-referenced. All code file paths, CRD field names, interface names, key patterns, and TTL values were verified against the implementation. Nice work.

[github-actions]

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