Add project ownership transfer endpoint

[Copilot]

Implements POST /v1/projects/{owner}/{project}/transfer-ownership to allow project owners to transfer ownership to another user.

Changes

Database

• Added TransferProjectOwnership SQL query updating projects.owner and users_projects atomically

API

• New endpoint with request model accepting new_owner_handle
• Returns old/new owner handles and project metadata
• Authorization: owner-only via existing auth middleware

Validation

• New owner must exist and differ from current owner
• Prevents conflicts: new owner cannot already have project with same handle
• Handles edge case: if new owner was previously shared (editor/reader), role is upgraded to owner

Transaction flow

// Atomic:
// 1. UPDATE projects SET owner = new_owner
// 2. DELETE FROM users_projects WHERE user = old_owner  
// 3. DELETE FROM users_projects WHERE user = new_owner (if exists)
// 4. INSERT INTO users_projects (new_owner, project_id, role='owner')

Tests
10 test cases covering: nonexistent project/user, self-transfer, unauthorized access, handle conflicts, successful transfers, and post-transfer access verification.

Documentation
Added README section with usage examples and behavioral notes (old owner loses access, embeddings remain intact).

Original prompt

Add an endpoint, model, handler, tests and documentation for a function that allows the owner of a project to transfer ownership to another user. Maybe the same (and optionally in one go) for other resources like definitions, instances etc...

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[awagner-mainz]

Looks good.