[ENG-715] feat: Add SAML SSO support

[twk3]

Add new feature from the upcoming release, SAML SSO support.

Users just need to provide their idp metadata file and a matching issuer id and SSO is enabled. Also includes support for restricting to specific domains, and auth signing. Tested with Okta.

Checklist

• PR title is Changelog-friendly: (fix|feat|chore): title [CSR-xxx]
• User-friendly and meaningful description of the changes
• Documentation was updated

Summary by CodeRabbit

• New Features
  • Added optional SAML 2.0 SSO for self-hosted and on-prem deployments, with a read-only SSO volume mount.
• Documentation
  • Updated quickstart, configuration, and backup/restore guides with SAML SSO setup steps, required environment variables, signing options, and restore procedures.
  • Added a dedicated SAML SSO walkthrough and linked it from the main getting started page.
• Chores
  • Synced on-prem Compose templates to mount the new SSO directory.
  • Bumped GitHub Actions to newer checkout/action versions.

[coderabbitai]

📝 Walkthrough

Walkthrough

This PR adds SAML SSO documentation and compose/configuration support for mounting SSO files, plus related quickstart and backup/restore updates. It also updates several GitHub Actions references and one Podman validation step.

Changes

SAML SSO Documentation and Volume Wiring

Layer / File(s)	Summary
Standalone SAML SSO setup guide docs/sso-saml.md	New guide covering SAML overview, IdP setup steps, required/optional env variables, signed AuthnRequests, cert rotation, and troubleshooting.
Documentation index links to SAML guide docs/README.md, docs/index.md	Adds “Enable SAML SSO” resource links pointing to the new guide.
Configuration reference and env example for SSO variables docs/configuration.md, on-prem/.env.example	Documents SSO environment variables and adds DC_SSO_VOLUME volume configuration entries.
Docker Compose SSO volume mounts on-prem/docker-compose.cache.yml, on-prem/docker-compose.database.yml, on-prem/docker-compose.full.yml, on-prem/templates/compose.currents.yml	Adds api service volume mounting DC_SSO_VOLUME read-only into /etc/currents/sso, and bumps version headers to 2026-01-26-001.
Quickstart and backup/restore procedures for SSO data docs/quickstart.md, docs/backup-restore.md	Updates Podman directory creation/permissions/SELinux steps and volume reference table for data/sso; adds backup and restore sections for SSO data.
GitHub Actions checkout and credentials updates .github/workflows/check-compose-sync.yml, .github/workflows/smoke-test-services.yml, .github/workflows/validate-compose.yml	Updates checkout to v5 in three workflows, updates AWS credentials to v6 in smoke-test-services, and adds a timeout in the Podman validation job.

Estimated code review effort: 2 (Simple) | ~10 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title is specific and matches the main change: adding SAML SSO support.
Description check	✅ Passed	The description is clear, user-friendly, and covers the key change plus docs update, matching the template's intent.

✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch feat/add-saml-sso

---

_{Comment @coderabbitai help to get the list of available commands.}

[coderabbitai]

Actionable comments posted: 3

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Inline comments:
In @.github/workflows/check-compose-sync.yml:
- Line 28: The checkout step in the check-compose-sync workflow is persisting
credentials unnecessarily. Update the existing actions/checkout@v5 step to
disable credential persistence by setting persist-credentials to false, since
this job only generates and diffs compose files and does not need authenticated
git access.

In @.github/workflows/smoke-test-services.yml:
- Line 29: The checkout step in this workflow currently keeps repository
credentials available even though the job does not perform any authenticated git
operations afterward. Update the actions/checkout usage in the smoke test job to
disable credential persistence by setting persist-credentials to false so the
job does not retain unnecessary git auth access.

In @.github/workflows/validate-compose.yml:
- Around line 22-23: The Checkout steps in both validation jobs are leaving
repository credentials persisted by default; update the actions/checkout usage
in the validate-compose workflow to disable persisted credentials. Add
persist-credentials set to false on each checkout step so the validation jobs
only fetch the repo without exposing the token to later steps.

🪄 Autofix (Beta)

Fix all unresolved CodeRabbit comments on this PR:

• Push a commit to this branch (recommended)
• Create a new PR with the fixes

---

ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: d218abf2-3111-46f5-9ce9-4dad993a3c75

📥 Commits

Reviewing files that changed from the base of the PR and between c1e3148 and 3a52439.

📒 Files selected for processing (3)

• .github/workflows/check-compose-sync.yml
• .github/workflows/smoke-test-services.yml
• .github/workflows/validate-compose.yml