Add entity manager foundation for ETL indexer

[raymondjacobson]

Scaffold the entity manager processing framework that will bring
discovery-provider parity to the ETL indexer. This PR establishes the
handler pattern, test infrastructure, database migrations, and local
runner -- no entity processing logic yet.

• processors/entity_manager/handler.go: Handler interface, Dispatcher,
  Params, EntityType/Action constants, ValidationError
• processors/entity_manager/validate.go: shared validators
  (ValidateSigner, ValidateHandle, ValidateBio, etc.) -- WILL BE REFACTORED!
• processors/entity_manager/testutil_test.go: setupTestDB, seeders,
  buildParams, mustHandle/mustReject assertion helpers
• processors/entity_manager/handler_test.go: dispatcher unit tests
• db/sql/migrations/0002: domain tables matching discovery-provider
  schema (users, tracks, playlists, follows, saves, reposts, routes,
  developer_apps, grants)
• indexer.go: wire dispatcher after raw ManageEntity append, add
  debug logging for tx payloads and completion
• examples/etl-local/main.go: standalone runner for local testing
  against production RPC + local Postgres
• FEATURE.md: updated architecture docs and entity roadmap

Testing

docker run -d --name etl-postgres \
  -e POSTGRES_PASSWORD=postgres \
  -e POSTGRES_DB=etl_local \
  -p 5432:5432 \
  postgres:16

go run ./examples/etl-local \
  --rpc https://rpc.audius.co \
  --db "postgres://postgres:postgres@localhost:5432/etl_local?sslmode=disable" \
  --start 21343648

Made-with: Cursor

[raymondjacobson]

• etl: add User Verify handler (0003 migration, verified_address) #150
• etl: add User Update handler (markNotCurrent, metadata merge) #149
• Add User Create entity manager handler #148
• Add entity manager foundation for ETL indexer #147 👈 (View in Graphite)
• main

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