Custom tenant metadata

[suchait007]

Feature 55 on the OSS roadmap, targeted at v0.2.

Extend the registry with arbitrary typed metadata fields.

Why it matters

The metadata jsonb column already round-trips a Map<String,String>. This is about typed accessors and validation on top of it.

Done means

• Existing untyped metadata keeps working
• Mutation-tested — break the implementation and confirm the test goes red
• No new non-optional dependency
• Docs updated under docs/
• Commits signed off (git commit -s)

---

New here? Read CONTRIBUTING.md, particularly The one rule that is not negotiable and What will be rejected. Comment before starting so nobody duplicates your work.