Use implementation ABI when decoding proxy calls

[Genmin]

What

Fixes #242 by letting the decoder use the stored implementation contract ABI when a proxy contract ABI does not contain the incoming selector.

The existing metadata flow already persists Contract.implementation and queues metadata retrieval for that implementation. This change wires that data into DataDecoderService.get_abi_function: proxy ABI selectors are still preferred, then implementation ABI selectors are checked, then the existing global selector fallback remains unchanged.

Why

For proxy contracts like the Sepolia example in #242, the proxy address can have metadata but the callable application selector is only present on the implementation ABI. Without this, decoding can fall through to a generic selector match and produce misleading argument names.

Tests

• uv run --with ruff ruff check app/services/data_decoder.py app/tests/services/test_data_decoder.py
• uv run --with ruff ruff format --check app/services/data_decoder.py app/tests/services/test_data_decoder.py
• ENV_FILE=.env.test DB_NAME=testdb uv run mypy app/services/data_decoder.py app/tests/services/test_data_decoder.py
• In-process decoder sanity check with a mocked proxy -> implementation lookup, including a misleading generic mint(address,uint256,uint256) fallback ABI

I also attempted the DB-backed regression directly:

• ENV_FILE=.env.test DB_NAME=testdb uv run pytest app/tests/services/test_data_decoder.py::TestDataDecoderService::test_decode_proxy_using_implementation_abi -q

That is blocked in my local environment because this repo expects local Postgres on localhost:5432, and Docker is not available here to start the test services.

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