[Codex] 1.4: Add database listing and creation methods

[taariq]

Parent: #28

Task

Add list_databases() and create_database() methods to src/serendb/client.rs.

File to Modify

src/serendb/client.rs

What to Add

1. list_databases(project_id, branch_id) - Lists all databases in a branch
2. create_database(project_id, branch_id, name) - Creates a new database

See docs/plans/api-key-flow-implementation.md Task 1.4 for full code.

How to Test

cargo test serendb::client::tests -- --nocapture

Commit Message

feat(serendb): Add database listing and creation methods

[taariq]

Finished Task 1.4: added SerenDB branch-scoped list_databases and create_database helpers, plus wired the interactive picker to the new signature. Verified via cargo test serendb::client::tests -- --nocapture.