feat(graph): add FastAPI route resolver

[sidsri14]

Summary

• detect FastAPI from pyproject.toml and requirements files
• extract stable route nodes for literal FastAPI and APIRouter decorators across the supported HTTP methods
• resolve route references only to unambiguous same-file Python functions or methods
• add a fixture and focused coverage for detection, extraction, registration, successful resolution, and unresolved edge cases

Closes #111

Validation

• npm run typecheck
• vitest run src/graph/__tests__/resolver-fastapi.test.ts (9 passed)
• npm run build
• full Vitest run: 377 passed; the remaining Windows-only failures reproduce on main and are caused by symlink permissions plus SQLite temporary-file locking