Seshat Storage

SQLite storage layer for Seshat. Owns ALL database interaction — no other crate touches SQL directly.

Responsibilities:

• Database lifecycle (Database::open, migrations, WAL mode)
• Repository traits and SQLite implementations for nodes, edges, files_ir, and branches
• FTS5 full-text search
• Automatic database backups
• Schema migrations via refinery (embed_migrations!)

Connection management: single writer (Arc<Mutex<Connection>>) with pooled read-only connections for concurrent queries (SQLite WAL mode).