docs(auth): define shared-host gateway architecture

[onutc]

Summary

• add a focused shared-host auth gateway architecture doc for Spritz
• make the preferred model explicit: one browser-facing auth gateway per public host
• update the existing auth and deployment docs to point at that architecture

Validation

• npx -y @simpledoc/simpledoc check
• git diff --check

[gitrank-connector]

👍 GitRank PR Analysis

Score: 5 points

Metric	Value
Component	Other (1× multiplier)
Severity	P3 - Low (5 base pts)
Final Score	5 × 1 = 5

Eligibility Checks

Check	Status
Issue/Bug Fix	❌
Fix Implementation	✅
PR Documented	✅
Tests	✅ (not required)
Lines Within Limit	✅

Impact Summary

This PR establishes a clear shared-host auth gateway architecture specification for Spritz deployments, making the preferred model explicit (one browser-facing auth gateway per public host). It updates existing documentation to reference this new architecture and provides detailed guidance on topology, route ownership, and validation procedures. The changes help operators understand and implement correct authentication patterns, reducing potential configuration errors.

Analysis Details

Component Classification: This PR is purely documentation - it adds and updates architectural documentation files without modifying any code, configuration, or business logic. Documentation changes fall under the OTHER category.

Severity Justification: This is a P3 (Low) contribution as it provides architectural guidance and documentation clarification without fixing a broken feature or addressing a critical issue. Documentation improvements are valuable but have minimal direct impact on system functionality.

Eligibility Notes: Tests are not required for this change type. This is pure documentation - adding a new architecture specification document (279 lines) and updating references in existing docs. No code, tests, or configuration changes are present. The PR is well-documented with clear description of changes and validation steps performed (simpledoc check, git diff check).

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