Keep the engine's state in SQLite, not Postgres

One process owns this database — the image has run a single uvicorn
worker for that reason since the four-engines bug — so a file beside the
flows is the honest shape for it, and it is what lets `fluksio serve`
need no infrastructure at all. Live values, node execution and the work
queue never came here anyway; what does is a rollup a minute at a time,
a row per cascade and the run history, and WAL keeps the readers going
while that one writer works.

DATA_DIR is now the one setting that moves everything an installation
keeps; the rest derive from it and the images still spell theirs out.
The schema is prepared in-process at startup, so the prestart service is
gone, and the ten Postgres-only revisions collapse into one portable
baseline.

Three things only worked because psycopg was casting for us: a token's
subject arriving as a string where the column is a UUID, `greatest`, and
`date_bin`. The timestamps needed a column type of their own — SQLite
stores no offset, and a naive datetime read back either raises against an
aware `now` or serialises as local time.

Postgres stays in the stack only for Umami, behind the analytics profile.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

backend/tests/flow/test_connector.py

@@ -146,7 +146,9 @@ def test_a_connector_is_discovered_from_its_entry_point(monkeypatch):
         def load(self):
             return Sensor
 
-    monkeypatch.setattr("fluksio.flow.plugins.entry_points", lambda group: [FakeEntry()])
+    monkeypatch.setattr(
+        "fluksio.flow.plugins.entry_points", lambda group: [FakeEntry()]
+    )
     try:
         assert load_plugins() == ["test_sensor"]
         assert NODE_TYPES["test_sensor"].plugin == "fluksio-connector-test 0.1.0"
@@ -166,7 +168,9 @@ def test_a_connector_written_for_another_contract_is_refused(monkeypatch):
         def load(self):
             return Outdated
 
-    monkeypatch.setattr("fluksio.flow.plugins.entry_points", lambda group: [FakeEntry()])
+    monkeypatch.setattr(
+        "fluksio.flow.plugins.entry_points", lambda group: [FakeEntry()]
+    )
     assert load_plugins() == []
     assert "outdated" not in NODE_TYPES
 
@@ -179,6 +183,8 @@ def test_a_connector_may_not_take_over_a_built_in_type(monkeypatch):
         def load(self):  # pragma: no cover - never reached
             raise AssertionError("should not be loaded")
 
-    monkeypatch.setattr("fluksio.flow.plugins.entry_points", lambda group: [FakeEntry()])
+    monkeypatch.setattr(
+        "fluksio.flow.plugins.entry_points", lambda group: [FakeEntry()]
+    )
     assert load_plugins() == []
     assert NODE_TYPES["mqtt"].plugin is None