Remove Memgraph: complete removal of graph database code and infrastructure

[hammertoe]

Summary

Remove all Memgraph (graph database) code and infrastructure from the codebase. The project no longer uses Memgraph and keeping it adds unnecessary complexity and maintenance burden.

Removal Plan

Phase 1: Tests First

• Add/adjust tests to reflect a Postgres-only system: no Memgraph client initialization, no Memgraph fixtures
• Update tests/test_database.py - remove Memgraph connection tests
• Update tests/test_bill_ingestion.py - remove Memgraph integration
• Update tests/test_bill_ingestor_unit.py - mock only what's needed
• Update tests/test_advanced_search.py - remove Memgraph mocks/usage
• Update tests/conftest.py - remove Memgraph from integration marker

Phase 2: Core Library Code

• Delete lib/db/memgraph_client.py
• Refactor api/search_api.py - remove Memgraph import, globals, startup, _get_memgraph()
• Refactor lib/advanced_search_features.py - Postgres-only, remove Memgraph-backed methods
• Refactor lib/processors/bill_ingestor.py - remove graph writes, Memgraph constructor
• Update lib/chat_agent.py - fix any call sites for changed signatures
• Delete lib/knowledge_graph/ingestor.py (Memgraph-only)
• Update lib/knowledge_graph/__init__.py
• Remove Memgraph config from lib/utils/config.py

Phase 3: Scripts

• Delete scripts/kg_sync_memgraph.py
• Remove Memgraph from scripts/ingest_knowledge_graph.py
• Remove Memgraph from scripts/migrate_transcripts.py
• Remove Memgraph from scripts/ingest_bills.py

Phase 4: Infrastructure & Config

• Remove pymgclient from requirements.txt
• Remove Memgraph env vars from .env.example
• Remove Memgraph service/volume from docker-compose.yml
• Remove Memgraph branches from db.sh

Phase 5: Documentation

• Remove Memgraph references from docs/KG_PLAN.md
• Remove Memgraph references from docs/CODE_MAP_AND_REVIEW.md
• Update frontend/README.md if endpoint behavior changes

Phase 6: Verification

• rg -ni \"memgraph|pymgclient|MEMGRAPH_\" returns no project references
• ruff check . passes
• mypy lib/ passes
• pytest tests/ -v passes
• docker-compose up -d starts only Postgres
• API startup has no Memgraph warnings

Files to Delete

• lib/db/memgraph_client.py
• scripts/kg_sync_memgraph.py
• lib/knowledge_graph/ingestor.py

Files to Modify

• api/search_api.py
• lib/advanced_search_features.py
• lib/processors/bill_ingestor.py
• lib/chat_agent.py
• lib/knowledge_graph/__init__.py
• lib/utils/config.py
• scripts/ingest_knowledge_graph.py
• scripts/migrate_transcripts.py
• scripts/ingest_bills.py
• tests/test_database.py
• tests/test_bill_ingestion.py
• tests/test_bill_ingestor_unit.py
• tests/test_advanced_search.py
• tests/conftest.py
• docker-compose.yml
• .env.example
• requirements.txt
• db.sh
• docs/KG_PLAN.md
• docs/CODE_MAP_AND_REVIEW.md