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/scripts/soak.py

@@ -53,10 +53,10 @@ COMPOSE_PROJECT = "fluksio-app"
 REDIS = "fluksio-redis"
 API = "fluksio-api"
 BROKER = "fluksio-app-mosquitto-1"
-DB = "fluksio-db"
+
 #: The only containers this harness may ever name. The first three are the
 #: chaos targets; the database is read and cleaned up, never stopped.
-ALLOWED = frozenset({REDIS, API, BROKER, DB})
+ALLOWED = frozenset({REDIS, API, BROKER})
 
 
 class HostSafety(RuntimeError):
@@ -805,25 +805,22 @@ def clear_redis() -> None:
 
 def clear_observability() -> None:
     """The collector wrote rollups, runs and events for the soak flows."""
-    names = "', '".join(FLOWS)
-    statement = "; ".join(
-        f"DELETE FROM {table} WHERE flow IN ('{names}')"
-        for table in ("metric_minute", "flow_run", "engine_event")
+    names = ", ".join(repr(flow) for flow in FLOWS)
+    script = "\n".join(
+        [
+            "import sqlite3",
+            "db = sqlite3.connect('/data/fluksio.db')",
+            *(
+                f"db.execute('DELETE FROM {table} WHERE flow IN ({names})')"
+                for table in ("metric_minute", "flow_run", "engine_event")
+            ),
+            "db.commit()",
+        ]
     )
     try:
-        docker(
-            DB,
-            "exec",
-            DB,
-            "psql",
-            "-q",
-            "-U",
-            settings.POSTGRES_USER,
-            "-d",
-            settings.POSTGRES_DB,
-            "-c",
-            statement,
-        )
+        # Through the api container: the database is a file on its volume, not
+        # a server of its own.
+        docker(API, "exec", API, "python", "-c", script)
         log.info("deleted observability rows for the soak flows")
     except Exception as exc:
         log.error("observability rows may be left behind: %s", exc)