fix: re-seed admin password when still unfinalized (must_change_password=True)

_ensure_admin_user was strict insert-if-missing: once a stale hash landed
in decnet.db (e.g. from a deploy that used a different DECNET_ADMIN_PASSWORD),
login silently 401'd because changing the env var later had no effect.

Now on startup: if the admin still has must_change_password=True (they
never finalized their own password), re-sync the hash from the current
env var. Once the admin sets a real password, we leave it alone.

Found via locustfile.py login storm — see tests/test_admin_seed.py.

Note: this commit also bundles uncommitted pool-management work already
present in sqlmodel_repo.py from prior sessions.

tests/test_admin_seed.py

@@ -0,0 +1,66 @@
+"""
+Tests for _ensure_admin_user env-drift self-healing.
+
+Scenario: DECNET_ADMIN_PASSWORD changes between runs while the SQLite DB
+persists on disk. Previously _ensure_admin_user was strictly insert-if-missing,
+so the stale hash from the first seed locked out every subsequent login.
+
+Contract: if the admin still has must_change_password=True (they never
+finalized their own password), the stored hash re-syncs from the env.
+Once the admin picks a real password, we never touch it.
+"""
+import pytest
+
+from decnet.web.auth import verify_password
+from decnet.web.db.sqlite.repository import SQLiteRepository
+
+
+@pytest.mark.asyncio
+async def test_admin_seeded_on_empty_db(tmp_path, monkeypatch):
+    monkeypatch.setattr("decnet.web.db.sqlmodel_repo.DECNET_ADMIN_PASSWORD", "first")
+    repo = SQLiteRepository(db_path=str(tmp_path / "t.db"))
+    await repo.initialize()
+    user = await repo.get_user_by_username("admin")
+    assert user is not None
+    assert verify_password("first", user["password_hash"])
+    assert user["must_change_password"] is True or user["must_change_password"] == 1
+
+
+@pytest.mark.asyncio
+async def test_admin_password_resyncs_when_not_finalized(tmp_path, monkeypatch):
+    db = str(tmp_path / "t.db")
+
+    monkeypatch.setattr("decnet.web.db.sqlmodel_repo.DECNET_ADMIN_PASSWORD", "first")
+    r1 = SQLiteRepository(db_path=db)
+    await r1.initialize()
+
+    monkeypatch.setattr("decnet.web.db.sqlmodel_repo.DECNET_ADMIN_PASSWORD", "second")
+    r2 = SQLiteRepository(db_path=db)
+    await r2.initialize()
+
+    user = await r2.get_user_by_username("admin")
+    assert verify_password("second", user["password_hash"])
+    assert not verify_password("first", user["password_hash"])
+
+
+@pytest.mark.asyncio
+async def test_finalized_admin_password_is_preserved(tmp_path, monkeypatch):
+    db = str(tmp_path / "t.db")
+
+    monkeypatch.setattr("decnet.web.db.sqlmodel_repo.DECNET_ADMIN_PASSWORD", "seed")
+    r1 = SQLiteRepository(db_path=db)
+    await r1.initialize()
+    admin = await r1.get_user_by_username("admin")
+    # Simulate the admin finalising their password via the change-password flow.
+    from decnet.web.auth import get_password_hash
+    await r1.update_user_password(
+        admin["uuid"], get_password_hash("chosen"), must_change_password=False
+    )
+
+    monkeypatch.setattr("decnet.web.db.sqlmodel_repo.DECNET_ADMIN_PASSWORD", "different")
+    r2 = SQLiteRepository(db_path=db)
+    await r2.initialize()
+
+    user = await r2.get_user_by_username("admin")
+    assert verify_password("chosen", user["password_hash"])
+    assert not verify_password("different", user["password_hash"])