Add ADR 31 about minimal API adoption

[justindbaur]

🎟️ Tracking

Relates to: ARCH-16

📔 Objective

Adds an ADR proposing that all new server endpoints be built using .NET minimal APIs in feature-scoped libraries, rather than continuing to add to the existing controller-based approach.

The ADR covers:

• Why now: .NET 10 closes the last significant feature gap (automatic validation), and we've already proven a smaller slices architecture in clients and sdk
• The required library shape: Add[Feature]Services + Map[Feature]Endpoints with a code example showing side-by-side usage with existing controllers
• Migration strategy: existing controllers migrated opportunistically, with a more aggressive timeline for endpoint-light projects (Icons, Notifications, Events)
• Trade-offs: mixed codebase during transition, and ActionFilterAttributes needing rewrite before affected controllers can be migrated

📸 Screenshots

[github-actions]

🤖 Bitwarden Claude Code Review

Overall Assessment: APPROVE

Reviewed the addition of ADR 0031 proposing adoption of .NET minimal APIs for new server endpoints in feature-scoped libraries. The new ADR follows the established frontmatter and section structure (Context, Considered options, Decision outcome, Consequences, Plan), auto-registers via <DocCardList /> so no sidebar changes are needed, and the illustrative C# example is consistent with prior decisions (e.g. TryAddScoped per ADR 0026). No security, correctness, or breaking-change concerns — this is a documentation-only, Proposed-status ADR.

Code Review Details

No findings.

[Hinton]

Do we want a formal ADR for the folder changes to server?

[justindbaur]

I have another ADR I want to write about breaking up Core that I think that would fit in better. I can write that first or I can just temporarily remove the src/Libraries part for now and this doc can go straight in src.

[justindbaur]

I've created #826

[theMickster]

Handful of suggestions to strengthen the prose in the document.
One noted significant concern about a mixed codebase of controllers of different types for an indefinite timeframe.

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