Implement Multi-Signature Wallet example and related backend mocks

[OSEH-svg]

closes #391
closes #194
closes #191
closes #204

This PR implements a minimal Multi-Signature Wallet Soroban contract as an example, and adds mock backend services used for development:

• examples/multisig: new Soroban contract with initialize/propose/approve/execute and tests.
• backend/src/services/cache.rs: mock MultiLevelCache (in-memory) with basic TTL.
• backend/src/services/transformers.rs: simple DataTransformer with normalization/extraction.
• backend/src/db/seeds.rs: improve error aggregation in run_all.

These are safe, self-contained additions intended to provide a working MVP and test harness. Further integration (Redis, production hooks) can be added in follow-ups.

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