Scaffold project, data model and artwork

Establish the LLeMbas foundation: FastAPI/Jinja/SQLite layout, the ORM
schema, and the original SVG identity.

Notable decisions, all recorded in comments at the point they matter:

- No Alembic. SQLite only, schema created at startup, so models carry a
  few columns nothing reads yet (Message.parent_id for branching,
  content_parts_json for multimodal turns). Adding them later to a live
  database without migrations is the painful path.
- Sessions are server-side rows keyed by a SHA-256 of the cookie value,
  not JWTs, so logout and bans revoke access immediately.
- Upstream API keys are Fernet-encrypted with a key derived from
  LEMBAS_SECRET_KEY. decrypt() fails soft to "" so rotating the secret
  degrades to re-entering keys rather than crashing the admin UI.
- Artwork is generated by scripts/build_artwork.py rather than hand-drawn
  per file: the mallorn leaf appears in the icon, favicon, lockup and
  banner, and one source is the only way those stay in sync. The wordmark
  is Source Serif 4 (OFL) converted to outlines, because a README banner
  cannot load a webfont and <text> would render in whatever serif the
  viewer happens to have.
- Icons live in a template partial, not assets/, because same-document
  <use href="#id"> is universally supported and the cross-document form
  is not.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/lembas/db/session.py

@@ -0,0 +1,102 @@
+"""Engine, session factory and startup schema creation."""
+
+from __future__ import annotations
+
+import logging
+from collections.abc import Iterator
+from contextlib import contextmanager
+
+from sqlalchemy import Engine, create_engine, event
+from sqlalchemy.orm import Session, sessionmaker
+
+from lembas.config import settings
+from lembas.db.base import Base
+
+log = logging.getLogger(__name__)
+
+_engine: Engine | None = None
+_SessionFactory: sessionmaker[Session] | None = None
+
+
+@event.listens_for(Engine, "connect")
+def _configure_sqlite(dbapi_connection, connection_record) -> None:  # noqa: ANN001
+    """Apply the pragmas SQLite needs to behave under a concurrent web server.
+
+    - WAL lets readers proceed while a write is in flight, which matters because
+      a streaming reply holds a write open for the length of the generation.
+    - foreign_keys is OFF by default in SQLite, so every ondelete= in the models
+      would be decoration without this.
+    - busy_timeout makes concurrent writers wait rather than fail instantly.
+    """
+    cursor = dbapi_connection.cursor()
+    cursor.execute("PRAGMA journal_mode=WAL")
+    cursor.execute("PRAGMA foreign_keys=ON")
+    cursor.execute("PRAGMA busy_timeout=5000")
+    cursor.execute("PRAGMA synchronous=NORMAL")
+    cursor.close()
+
+
+def get_engine() -> Engine:
+    global _engine
+    if _engine is None:
+        settings.ensure_dirs()
+        _engine = create_engine(
+            f"sqlite:///{settings.db_path}",
+            # FastAPI runs sync endpoints in a threadpool, so a connection can
+            # legitimately be used from a thread other than the one that made it.
+            connect_args={"check_same_thread": False},
+            echo=False,
+            future=True,
+        )
+    return _engine
+
+
+def get_session_factory() -> sessionmaker[Session]:
+    global _SessionFactory
+    if _SessionFactory is None:
+        _SessionFactory = sessionmaker(
+            bind=get_engine(),
+            autoflush=False,
+            expire_on_commit=False,
+        )
+    return _SessionFactory
+
+
+def init_db() -> None:
+    """Create any missing tables.
+
+    This is ``CREATE TABLE IF NOT EXISTS`` only -- it never alters an existing
+    table. There is no migration tool in this project by design, so changing a
+    column on a model requires migrating the database by hand.
+    """
+    import lembas.db.models  # noqa: F401  (registers tables on the metadata)
+
+    Base.metadata.create_all(bind=get_engine())
+    log.debug("schema ensured at %s", settings.db_path)
+
+
+@contextmanager
+def session_scope() -> Iterator[Session]:
+    """Transactional scope for background work and CLI commands.
+
+    Request handlers should use the `db` dependency in lembas.api.deps instead.
+    """
+    factory = get_session_factory()
+    session = factory()
+    try:
+        yield session
+        session.commit()
+    except Exception:
+        session.rollback()
+        raise
+    finally:
+        session.close()
+
+
+def reset_engine() -> None:
+    """Drop cached engine/factory. Used by tests to rebind to a temp database."""
+    global _engine, _SessionFactory
+    if _engine is not None:
+        _engine.dispose()
+    _engine = None
+    _SessionFactory = None