tests: extract _build_session_factory to a shared conftest fixture

The same per-test sqlite-engine setup was duplicated across 14 test
files (~30 lines each). Consolidated into a single async fixture
`db_factory` in tests/conftest.py; tests now take db_factory as a
parameter and use `async with db_factory() as session` directly.

No behaviour change — same function-scope, same in-memory schema
created via Base.metadata.create_all, same app.db._engine /
_session_factory rebinding so module-level helpers see the test
engine. Just ~420 lines of boilerplate removed.

tests/test_llm_csv_parser.py

@@ -4,26 +4,6 @@ from __future__ import annotations
 import pytest
 
 
-def _build_session_factory(tmp_path):
-    """Spin up a fresh in-memory schema and return (engine, factory).
-    Matches the pattern used in tests/test_referral_conversion.py."""
-    from sqlalchemy.ext.asyncio import async_sessionmaker, create_async_engine
-
-    from app import db as db_mod
-    from app.db import Base
-    import app.models  # noqa: F401 — registers models on Base.metadata
-
-    engine = create_async_engine(f"sqlite+aiosqlite:///{tmp_path}/csv.db")
-    factory = async_sessionmaker(engine, expire_on_commit=False)
-    db_mod._engine = engine
-    db_mod._session_factory = factory
-
-    async def _setup():
-        async with engine.begin() as conn:
-            await conn.run_sync(Base.metadata.create_all)
-
-    return engine, factory, _setup
-
 
 def test_csv_format_template_model_columns():
     """Model exposes every column the spec requires, with correct types."""
@@ -310,7 +290,7 @@ async def test_extract_mapping_via_llm_provider_failure_wraps():
         await _extract_mapping_via_llm(fake_client, ["Symbol"], [["AAPL"]])
 
 
-async def test_parse_with_llm_cache_miss_inserts_template(tmp_path):
+async def test_parse_with_llm_cache_miss_inserts_template(db_factory):
     from unittest.mock import AsyncMock
     from sqlalchemy import select
 
@@ -318,8 +298,7 @@ async def test_parse_with_llm_cache_miss_inserts_template(tmp_path):
     from app.services.llm_csv_parser import parse_with_llm
     from app.services.openrouter import LogResult
 
-    _, factory, setup = _build_session_factory(tmp_path)
-    await setup()
+    factory = db_factory
 
     raw = (
         b"Symbol,Quantity,Avg Price,Currency\n"
@@ -356,7 +335,7 @@ async def test_parse_with_llm_cache_miss_inserts_template(tmp_path):
     assert not hasattr(tmpl, "user_id"), "sample row must not be linked to a user"
 
 
-async def test_parse_with_llm_cache_hit_skips_llm(tmp_path):
+async def test_parse_with_llm_cache_hit_skips_llm(db_factory):
     from unittest.mock import AsyncMock
     from sqlalchemy import select
 
@@ -364,8 +343,7 @@ async def test_parse_with_llm_cache_hit_skips_llm(tmp_path):
     from app.models import CsvFormatTemplate
     from app.services.llm_csv_parser import _fingerprint, parse_with_llm
 
-    _, factory, setup = _build_session_factory(tmp_path)
-    await setup()
+    factory = db_factory
 
     headers = ["Symbol", "Quantity", "Avg Price", "Currency"]
     fp = _fingerprint(headers)
@@ -411,7 +389,7 @@ async def test_parse_with_llm_cache_hit_skips_llm(tmp_path):
     assert rows[0].use_count == 2
 
 
-async def test_parse_with_llm_stale_mapping_raises_but_does_not_evict(tmp_path):
+async def test_parse_with_llm_stale_mapping_raises_but_does_not_evict(db_factory):
     from unittest.mock import AsyncMock
     from sqlalchemy import select
 
@@ -419,8 +397,7 @@ async def test_parse_with_llm_stale_mapping_raises_but_does_not_evict(tmp_path):
     from app.models import CsvFormatTemplate
     from app.services.llm_csv_parser import LLMParseError, _fingerprint, parse_with_llm
 
-    _, factory, setup = _build_session_factory(tmp_path)
-    await setup()
+    factory = db_factory
 
     headers = ["Symbol", "Quantity"]
     fp = _fingerprint(headers)
@@ -452,7 +429,7 @@ async def test_parse_with_llm_stale_mapping_raises_but_does_not_evict(tmp_path):
     assert len(rows) == 1
 
 
-async def test_parse_portfolio_route_falls_through_to_llm(tmp_path, monkeypatch):
+async def test_parse_portfolio_route_falls_through_to_llm(db_factory, monkeypatch):
     """End-to-end: T212 parser raises CSVImportError, LLM fallback runs,
     response shape matches the existing JSON contract."""
     from io import BytesIO
@@ -461,8 +438,7 @@ async def test_parse_portfolio_route_falls_through_to_llm(tmp_path, monkeypatch)
 
     from fastapi import UploadFile
 
-    _, factory, setup = _build_session_factory(tmp_path)
-    await setup()
+    factory = db_factory
 
     import app.services.llm_csv_parser as mod
     from app.services.openrouter import LogResult