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).

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