Back portfolio and disputes endpoints with indexed reads

[220nightmore-spec]

Closes #1712.

Summary

• remove the hard-coded MOCK_PORTFOLIO and MOCK_DISPUTES controller arrays
• derive portfolio entries from indexed bids for the requested investor, joined with indexed/persisted invoices for status mapping
• read disputes from the derived-table store and keep invoice-id filtering plus cursor pagination
• add listBids() and listDisputes() to the derived-table store API for in-memory and file-backed stores
• keep existing response wrappers: { data, next_cursor, has_more }
• add focused Jest coverage for persistence-backed portfolio derivation, empty investor portfolios, pagination, and dispute filtering

Validation

• node .\node_modules\jest\bin\jest.js src\tests\portfolio-disputes-persistence.test.ts --runInBand --forceExit
• git diff --check
• rg "MOCK_PORTFOLIO|MOCK_DISPUTES" backend/src backend/tests backend/openapi.yaml returned no matches

Notes

• src/tests/cache-headers.endpoints.test.ts currently fails before assertions because importing app reaches the existing NotificationService / CircuitBreaker is not defined issue.
• Full tsc --noEmit --pretty false is currently blocked by an existing syntax error in src/index.ts (TS1005 ')' expected / '}' expected).