P2 slice 5: files in and out

Files reach the books through the server: uploads by hash and size,
then filed; exports (the CPA package, the 1099 worksheet, W-9s) handed
over as downloads from the server, never read off disk by the agent.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T3gXMVt7XJPi6ens1or9Dj

crates/ledger-store/src/lib.rs

@@ -32,6 +32,7 @@ pub mod root;
 pub mod schema;
 pub mod snapshot;
 pub mod store;
+pub mod uploads;
 pub mod viewer;
 
 pub use blob::BlobStore;