fix: implement incomplete features and apply linter fixes
- Fixed TransferDirection import error in ethereum.rs tests
- Implemented math.tanh function for Flutter tensor operations
- Added DocumentStore CRUD methods (find_by_id, update_by_id, delete_by_id)
- Implemented database gateway handlers (get/update/delete document)
- Applied cargo fix across all crates to resolve unused imports/variables
- Reduced warnings from 320+ to 68 (remaining are architectural)
Affected crates: synor-database, synor-bridge, synor-compute,
synor-privacy, synor-verifier, synor-hosting, synor-economics
7e3bbe569c