T-229: V003 migration terminal_sessions table

[mpiton]

Goal

Add V003__terminal_sessions.sql migration creating the terminal_sessions table per ARCHI §11.4 so future sprints can serialise xterm scrollback + metadata across restarts. This sprint only creates the table; the actual write path lands in a follow-up issue (see T-237 risks).

Columns mirror ARCHI sample: id, project_id (FK projects), cwd, shell_program, cols, rows, serialize_buffer (BLOB, optional), created_at, updated_at.

Acceptance criteria

• New file src-tauri/src/infrastructure/persistence/migrations/V003__terminal_sessions.sql
• CREATE TABLE IF NOT EXISTS terminal_sessions with all columns + indexes (idx_terminal_sessions_project)
• project_id foreign key references projects(id)
• pool::migrate runs V003 in-order without panicking on a fresh DB
• Existing DBs receive V003 idempotently (no drop, no data loss)
• Migration smoke test in pool.rs runs all migrations against in-memory libsql and asserts terminal_sessions exists
• No regression on V001/V002 tests

Files to create/modify

• create src-tauri/src/infrastructure/persistence/migrations/V003__terminal_sessions.sql
• modify src-tauri/src/infrastructure/persistence/pool.rs (migration list)

References

• ARCHI.md §8 Modèle de données
• ARCHI.md §11.4 Persistence
• Existing V001/V002 patterns

Dependencies

None — can run in parallel with T-227.