docs: consolidate overlapping docs and archive shipped plans

- Merge STORAGE_ARCHITECTURE.md + STORAGE_QUICK_SETUP.md → STORAGE.md (setup-first, then architecture)
- Merge OIDC_SETUP.md + OIDC_ARCH.md → OIDC.md (setup-first, then architecture)
- Merge orphaned adding-a-new-database-provider.md into DATABASE_PROVIDERS.md
- Archive shipped implementation plans: AI_PLAN.md, POSTGRES_METRICS.md → docs/archived/
- Update README OIDC links to the consolidated OIDC.md

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: cevheri

README.md

@@ -600,8 +600,7 @@ extraEnvFrom:
 | [DeepWiki](https://deepwiki.com/libredb/libredb-studio) | AI-powered documentation — always up-to-date with the codebase |
 | [SonarCloud](https://sonarcloud.io/project/overview?id=libredb_libredb-studio) | Code quality, security analysis, and technical debt tracking |
 | [API Docs](docs/API_DOCS.md) | Complete REST API reference |
-| [OIDC Setup Guide](docs/OIDC_SETUP.md) | SSO configuration for Auth0, Keycloak, Okta, Azure AD, Zitadel |
-| [OIDC Architecture](docs/OIDC_ARCH.md) | OIDC subsystem internals, security model, extension points |
+| [OIDC SSO](docs/OIDC.md) | SSO setup (Auth0, Keycloak, Okta, Azure AD, Zitadel, Google) + subsystem internals & security model |
 | [Theming Guide](docs/THEMING.md) | CSS theming, dark mode, and styling customization |
 | [Architecture](docs/ARCHITECTURE.md) | System architecture and design patterns |