[M1] Document the MindSpace V2 foundation

[rodrigolinhas]

Goal

Update documentation after the real foundation exists.

Expected Scope

• V2 repository/backend layout.
• Local PostgreSQL startup instructions.
• Environment variables and configuration reference.
• Migration commands and workflow.
• Running V2 in development and production.
• Running automated tests.
• Accessing OpenAPI documentation.
• Coexistence with V1 during migration.

Update:

• docs/architecture-v2.md
• docs/migration-v1-v2.md

Do not document planned behavior as implemented behavior.

Acceptance Criteria

• architecture-v2.md reflects actual M1 code.
• Setup instructions are reproducible.
• Migration document records M1 decisions.
• Persistence choice and rationale are recorded.
• Remaining open decisions remain explicitly open.

Dependencies

All substantive M1 implementation issues (Issues #6 through #13).

Suggested PR

PR 5 — M1 final documentation