Enable SQLite WAL mode + cascading deletes

[ryuhmanov-m]

Parent issue: #23

Context

The database currently uses default journal mode and lacks cascading deletes, which can leave orphan records.

Scope

1. Enable WAL mode

Add these PRAGMAs on connection init:

PRAGMA journal_mode=WAL;
PRAGMA busy_timeout=5000;

This allows concurrent reads during writes and prevents "database is locked" errors.

2. Add cascading deletes

Ensure that when a conversation is deleted, all related messages and tool calls are cleaned up. Either:

• Add ON DELETE CASCADE to foreign keys in migration
• Or add application-level cleanup in delete_conversation()

3. Wrap reindex in a transaction

Currently reindex_vault deletes all chunks then inserts new ones. If the process crashes mid-way, data is lost. Wrap in a single transaction:

let tx = conn.transaction()?;
tx.execute("DELETE FROM chunks WHERE vault_path = ?", [&vault_path])?;
// ... insert new chunks ...
tx.commit()?;

Files to look at

• src-tauri/src/adapters/db/ — database module
• Migration files in src-tauri/migrations/