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/models/__init__.py

@@ -0,0 +1,45 @@
+"""All ORM models.
+
+Importing this package registers every table on ``Base.metadata``, which is
+what ``init_db()`` relies on to create the schema at startup. Any new model
+module must be imported here or its table will silently never be created.
+"""
+
+from lembas.db.models.chat import (
+    ROLE_ASSISTANT,
+    ROLE_SYSTEM,
+    ROLE_TOOL,
+    ROLE_USER,
+    Chat,
+    Folder,
+    Message,
+)
+from lembas.db.models.connection import Connection, Model
+from lembas.db.models.setting import Setting
+from lembas.db.models.user import (
+    ROLE_ADMIN,
+    ROLE_PENDING,
+    Group,
+    Session,
+    User,
+    user_groups,
+)
+
+__all__ = [
+    "ROLE_ADMIN",
+    "ROLE_ASSISTANT",
+    "ROLE_PENDING",
+    "ROLE_SYSTEM",
+    "ROLE_TOOL",
+    "ROLE_USER",
+    "Chat",
+    "Connection",
+    "Folder",
+    "Group",
+    "Message",
+    "Model",
+    "Session",
+    "Setting",
+    "User",
+    "user_groups",
+]