feat: link news to on-chain entities

[zakkiyyat]

Summary

• add deterministic on-chain entity linker with stable project/asset IDs
• persist article entity links for backend consumption
• expose links on news API responses and add precision evaluation coverage

Tests

• .venv/bin/python -m pytest tests/test_onchain_entity_persistence.py tests/test_onchain_entity_linker.py tests/test_ner_service.py -q
• .venv/bin/python -m py_compile src/analytics/onchain_entity_linker.py src/analytics/ner_service.py src/analytics/init.py src/db/models.py src/db/postgres_service.py src/db/init.py src/api/server.py alembic/versions/003_add_onchain_entity_links.py tests/test_onchain_entity_linker.py tests/test_onchain_entity_persistence.py

Closes #739

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