Switch to GCS Default & Remove Firestore

[prosdev]

Goal

Switch default storage to GCS and remove Firestore implementation after GCS is proven working.

Depends on: #24 (GCS + BigQuery Storage Implementation)

Rationale

• Firestore doesn't match production CDP patterns
• No production CDP uses Firestore for event storage
• Supporting multiple backends adds unnecessary complexity
• GCS → BigQuery is industry standard

Tasks

1. Switch default to GCS

   • Change EVENTKIT_EVENT_STORE default to "gcs"
   • Update documentation to use GCS
   • Update .env.example

2. Remove Firestore implementation

   • Delete src/eventkit/stores/firestore.py
   • Delete Firestore tests
   • Remove Firestore emulator from Docker Compose
   • Keep EventStore/ErrorStore Protocols (for pluggability)

3. Simplify configuration

   • Remove FIRESTORE_DATABASE setting
   • Remove multi-backend factory switching
   • Fix EventLoader defaults for GCS

4. Update documentation

   • Remove Firestore references
   • Document extension points (S3, Azure examples)

5. Verify tests pass

   • All unit tests pass
   • All integration tests pass (GCS emulator)

Timeline

Estimated: 0.5 days (cleanup after GCS proven)

Acceptance Criteria

• GCS is default storage backend
• Firestore implementation deleted
• Protocols kept for pluggability (EventStore, ErrorStore)
• All tests passing
• Documentation updated
• Codebase simplified