docs: add ADR 005 — Organization entity and assistant derivation

[martinydeAI]

Summary

Adds ADR 005 recording the Organization entity, its metadata,
its relations to User and Assistant, and the derivation flow
that pre-fills framework + language model on new assistants from
the creator's organisation.

Closes #65.

Highlights

• Organization is a first-class entity with name, e-mail
  domain(s), defaultFramework, defaultLanguageModel.
• User → Organization is M:1, set at signup time by
  matching the user's e-mail domain against
  Organization.emailDomains.
• Assistant → Organization is M:N — multiple kommuner can
  use the same assistant.
• Assistant.framework and Assistant.languageModel are stored
  on the assistant as a snapshot at creation, not joined
  through the org at read time. An org switching its default LM
  next year must not silently rewrite last year's catalogue.

Interaction with ADR 004

ADR 004's REGISTRATION_ALLOWED_EMAIL_DOMAINS env var is
superseded by Organization.emailDomains once Organization
lands. Until that happens, the env var stays as the bootstrap
mechanism. ADR 004 itself doesn't need re-deciding — this is
exactly the "graduate to a Domain entity" follow-up it
anticipated.

Implementation outline

The ADR sketches three downstream pieces (issues to file alongside
or shortly after):

1. Organization entity + migration + User.organization_id FK.
2. Registration allow-list switchover (/register validates
   against registered orgs instead of the env var).
3. Assistant entity (Define assistant data model #14) gets framework, languageModel, and
   the organizations M:N; the creation flow derives defaults
   from the creator's org.

Test plan

• task coding-standards-markdown-check → clean.
• Reviewer confirms snapshot-vs-live-derive is the desired
  durability story (i.e. existing assistants keep their
  original framework / LM if an org changes its default
  later).
• Reviewer confirms one-org-per-user is acceptable as the
  starting model (consultants / cross-org users would need a
  future M:N).

Closes #65

🤖 Generated with Claude Code

[martinyde]

In my opinion this makes sense.

We will however need a crud setup for administering the entity. The entities can also be maintained through an API, if the project owner has a centralized data source of orgnaizations and their AI setup, or this site could act as data source and expose an API of Organizations and their AI setup.

The right approach depends on how the project owner wants their flow of data