feat(pipeline): implement parliamentary embedding pipeline MVP

- Add 4 migration files: mp_votes, mp_metadata, svd_vectors, fused_embeddings
- Extend database.py with 5 new helper methods and table init
- Add pipeline/ package: extract_mp_votes, fetch_mp_metadata, text_pipeline,
  svd_pipeline (with Procrustes alignment), fusion
- Add full test suite (17 tests) covering all pipeline modules and migrations
- Fix Procrustes alignment bug: scipy scale is a norm value, not a multiplier
- Fix DuckDB date type handling in test assertions (datetime.date vs string)
- Remove duckdb.py shim; tests now run against real duckdb + scipy via uv

Ref: thoughts/shared/plans/2026-03-21-parliamentary-embedding-pipeline-plan.md

tests/utils/migration_fixtures.py

@@ -0,0 +1,66 @@
+"""
+Test helper fixtures for database migrations.
+
+Provides a pytest fixture `test_db` that inspects the environment variable
+`TEST_DB_URL` to decide what to yield:
+
+- If `TEST_DB_URL` is not set, the fixture yields None. This allows tests to
+  be skipped or operate in a no-database mode in CI or local runs where a
+  test database is not available.
+- If `TEST_DB_URL` is set and starts with "sqlite", an sqlite3 connection is
+  created via `sqlite3.connect` and yielded. The connection is closed after
+  the test completes.
+
+Decision: keep this fixture lightweight and focused on sqlite for local
+smoke-testing. If other database backends are needed later, expand this
+fixture accordingly.
+"""
+
+from typing import Optional
+import os
+import sqlite3
+
+import pytest
+
+
+@pytest.fixture
+def test_db():
+    """Yield a test database connection or None.
+
+    Behavior:
+    - If TEST_DB_URL is not set in the environment, yield None.
+    - If TEST_DB_URL is set and begins with 'sqlite', open an sqlite3
+      connection and yield it. The connection will be closed when the test
+      finishes.
+    """
+    url = os.environ.get("TEST_DB_URL")
+    if not url:
+        yield None
+        return
+
+    # Only support sqlite URLs in this lightweight fixture.
+    if url.startswith("sqlite"):
+        # For sqlite URLs, accept either a bare file path or a file:// style
+        # URL. sqlite3.connect handles file paths; if a file:// prefix is
+        # present, strip it.
+        path = url
+        if path.startswith("sqlite:///"):
+            # sqlite:///path => /path
+            path = path[len("sqlite:///") :]
+        elif path.startswith("sqlite://"):
+            path = path[len("sqlite://") :]
+
+        conn = sqlite3.connect(path)
+        try:
+            yield conn
+        finally:
+            try:
+                conn.close()
+            except Exception:
+                # Best-effort close; tests shouldn't fail on close errors.
+                pass
+        return
+
+    # Unknown or unsupported TEST_DB_URL scheme — yield None to keep tests
+    # tolerant in environments where the fixture can't create a connection.
+    yield None