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/fluksio/alembic/env.py

@@ -1,4 +1,3 @@
-import os
 from logging.config import fileConfig
 
 from alembic import context
@@ -10,7 +9,9 @@ config = context.config
 
 # Interpret the config file for Python logging.
 # This line sets up loggers basically.
-fileConfig(config.config_file_name)
+# Driven from `fluksio.core.db` there is no ini file to read logging from.
+if config.config_file_name is not None:
+    fileConfig(config.config_file_name)
 
 # add your model's MetaData object here
 # for 'autogenerate' support
@@ -20,9 +21,23 @@ fileConfig(config.config_file_name)
 
 from fluksio.models import SQLModel  # noqa
 from fluksio.core.config import settings # noqa
+from fluksio.core.types import UTCDateTime  # noqa
 
 target_metadata = SQLModel.metadata
 
+
+def render_item(type_, obj, autogen_context):
+    """Write the decorator out as the type it wraps.
+
+    A revision naming `UTCDateTime` would have to import from the app, which
+    ties a migration to a version of it. What it stores is a timezone-aware
+    timestamp, so that is what the revision says.
+    """
+    if type_ == "type" and isinstance(obj, UTCDateTime):
+        autogen_context.imports.add("import sqlalchemy as sa")
+        return "sa.DateTime(timezone=True)"
+    return False
+
 # other values from the config, defined by the needs of env.py,
 # can be acquired:
 # my_important_option = config.get_main_option("my_important_option")
@@ -61,6 +76,13 @@ def run_migrations_online():
     and associate a connection with the context.
 
     """
+    # `fluksio.core.db.migrate` hands us the connection it is already inside;
+    # the CLI and `alembic` on the command line make their own.
+    connection = config.attributes.get("connection")
+    if connection is not None:
+        _run(connection)
+        return
+
     configuration = config.get_section(config.config_ini_section)
     configuration["sqlalchemy.url"] = get_url()
     connectable = engine_from_config(
@@ -70,12 +92,20 @@ def run_migrations_online():
     )
 
     with connectable.connect() as connection:
-        context.configure(
-            connection=connection, target_metadata=target_metadata, compare_type=True
-        )
+        _run(connection)
 
-        with context.begin_transaction():
-            context.run_migrations()
+
+def _run(connection):
+    context.configure(
+        connection=connection,
+        target_metadata=target_metadata,
+        compare_type=True,
+        render_item=render_item,
+        # SQLite cannot ALTER much: batch mode rebuilds the table instead.
+        render_as_batch=connection.dialect.name == "sqlite",
+    )
+    with context.begin_transaction():
+        context.run_migrations()
 
 
 if context.is_offline_mode():