fix(migrations): honor configured vector extension before pgvector bootstrap

[r266-tech]

Summary

• read HINDSIGHT_API_VECTOR_EXTENSION before migration extension bootstrap
• keep the legacy pgvector public-schema relocation path only for the pgvector backend
• let non-pgvector backends use the shared bootstrap dispatcher before schema migrations run

Fixes #2423.

Tests

• uv run pytest tests/test_vector_index.py -q -k 'migration_bootstrap or bootstrap_extension_scann'
• uv run ruff check hindsight_api/migrations.py
• git diff --check

Review

• Codex adversarial review: approve