fix(migrations): schema base 000 per DB vuoti, stamp head dopo create_all (#6)

upgrade head partiva dalla 001, che sa solo aggiungere colonne a tabelle
che assume gia' esistenti: su un DB davvero vuoto falliva subito. Nuova
000_base_schema.py (revision "000_base", down_revision None) crea lo
schema pre-001 esplicitamente (create_table, non Base.metadata.create_all,
che porterebbe gia' le colonne di 001-013 e farebbe fallire 001 su
"Duplicate column"). No-op se la tabella recipes esiste gia'
(sa.inspect(bind).has_table("recipes")). 001 ora ha down_revision =
"000_base".

setup.py: init-db e reset-db, dopo create_all/drop_all+create_all,
eseguono l'equivalente di "alembic stamp head" (nuovo _stamp_head(),
command.stamp con la config di src/backend/migrations/alembic.ini) cosi'
un DB nato da create_all non ritenta le migrazioni 001-013 al prossimo
deploy.

Test: catena upgrade head su SQLite vuoto, tabelle/colonne confrontate
con Base.metadata; DB con recipes gia' nello stato pre-001 -> upgrade
head non fallisce (000 no-op). _stamp_head si appoggia a
migrations/env.py, che costruisce una connessione propria da
settings.database_url (non modificabile dai test tramite l'engine
patchato di setup.py): nei test di setup.py e' quindi no-op via
monkeypatch, cosi' init-db/reset-db continuano a girare su SQLite senza
toccare un MySQL vero.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/backend/tests/test_migrations.py

@@ -0,0 +1,109 @@
+"""Tests for the Alembic migration chain, and specifically for
+000_base_schema.py (issue #6): a genuinely empty database has no starting
+point before it, since 001-013 all assume tables 001 was written against
+already exist.
+
+These run the real `alembic upgrade head` (via `alembic.command`), not the
+async app engine: `migrations/env.py` builds its own engine from
+`settings.database_url`, so `Settings.database_url` is patched at the class
+level (a plain instance monkeypatch cannot override a `@property`) to point
+at a throwaway SQLite file for the duration of each test.
+"""
+from pathlib import Path
+
+import pytest
+from alembic import command
+from alembic.config import Config
+from sqlalchemy import create_engine, inspect as sa_inspect, text
+
+from src.backend.config import Settings
+from src.backend.database import Base
+
+_ALEMBIC_INI = (
+    Path(__file__).resolve().parents[1] / "migrations" / "alembic.ini"
+)
+
+
+@pytest.fixture
+def sqlite_database_url(tmp_path, monkeypatch):
+    """Point settings.database_url (and therefore migrations/env.py) at a
+    throwaway SQLite file, and return its async URL."""
+    db_path = tmp_path / "migration_test.db"
+    async_url = f"sqlite+aiosqlite:///{db_path}"
+    monkeypatch.setattr(Settings, "database_url", property(lambda self: async_url))
+    return db_path, async_url
+
+
+def _alembic_config() -> Config:
+    return Config(str(_ALEMBIC_INI))
+
+
+def _inspect(db_path: Path):
+    engine = create_engine(f"sqlite:///{db_path}")
+    try:
+        return sa_inspect(engine)
+    finally:
+        engine.dispose()
+
+
+class TestUpgradeHeadFromEmpty:
+    """Full chain on a genuinely empty SQLite database (#6)."""
+
+    def test_upgrade_head_succeeds_on_empty_database(self, sqlite_database_url):
+        db_path, _ = sqlite_database_url
+        command.upgrade(_alembic_config(), "head")  # must not raise
+
+        insp = _inspect(db_path)
+        assert set(Base.metadata.tables) <= set(insp.get_table_names())
+
+    def test_tables_and_columns_match_base_metadata(self, sqlite_database_url):
+        """Every table and column ORM models declare must exist after
+        `upgrade head` - the exact assertion the analysis asks for."""
+        db_path, _ = sqlite_database_url
+        command.upgrade(_alembic_config(), "head")
+
+        insp = _inspect(db_path)
+        db_tables = set(insp.get_table_names())
+
+        for table_name, table in Base.metadata.tables.items():
+            assert table_name in db_tables, f"missing table {table_name}"
+            db_columns = {c["name"] for c in insp.get_columns(table_name)}
+            model_columns = {c.name for c in table.columns}
+            missing = model_columns - db_columns
+            assert not missing, f"{table_name} missing columns: {missing}"
+
+
+class TestBaseMigrationNoopsOnExistingSchema:
+    """000_base_schema.py does nothing when `recipes` already exists (#6)."""
+
+    def test_upgrade_head_noops_000_when_pre_001_schema_already_there(
+        self, sqlite_database_url
+    ):
+        db_path, _ = sqlite_database_url
+        cfg = _alembic_config()
+
+        # Build the pre-001 schema and record it as migrated.
+        command.upgrade(cfg, "000_base")
+        insp = _inspect(db_path)
+        assert "recipes" in insp.get_table_names()
+        assert "image_path" not in {
+            c["name"] for c in insp.get_columns("recipes")
+        }
+
+        # Erase the tracking: alembic now believes nothing has run, exactly
+        # like a database that was born from Base.metadata.create_all() and
+        # never stamped. Running 000's upgrade() again on a database that
+        # already has `recipes` must be a no-op, not a "table already
+        # exists" failure.
+        sync_engine = create_engine(f"sqlite:///{db_path}")
+        with sync_engine.begin() as conn:
+            conn.execute(text("DELETE FROM alembic_version"))
+        sync_engine.dispose()
+
+        command.upgrade(cfg, "head")  # must not raise (no duplicate table)
+
+        insp = _inspect(db_path)
+        assert set(Base.metadata.tables) <= set(insp.get_table_names())
+        assert "image_path" in {
+            c["name"] for c in insp.get_columns("recipes")
+        }