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/services/crypto.py

@@ -0,0 +1,58 @@
+"""Symmetric encryption for secrets stored in the database.
+
+Only upstream API keys use this today. The key is derived from
+``LEMBAS_SECRET_KEY`` rather than stored separately, which means rotating that
+variable makes every stored API key unreadable -- decrypt() returns "" rather
+than raising, so the app degrades to "re-enter your keys" instead of crashing.
+"""
+
+from __future__ import annotations
+
+import base64
+import hashlib
+import logging
+from functools import lru_cache
+
+from cryptography.fernet import Fernet, InvalidToken
+
+from lembas.config import settings
+
+log = logging.getLogger(__name__)
+
+
+@lru_cache
+def _fernet() -> Fernet:
+    # Fernet requires a 32-byte urlsafe-base64 key; SECRET_KEY is free-form text.
+    digest = hashlib.sha256(settings.secret_key.encode("utf-8")).digest()
+    return Fernet(base64.urlsafe_b64encode(digest))
+
+
+def encrypt(plaintext: str) -> str:
+    """Encrypt a secret. Empty input stays empty -- keyless endpoints are valid."""
+    if not plaintext:
+        return ""
+    return _fernet().encrypt(plaintext.encode("utf-8")).decode("ascii")
+
+
+def decrypt(ciphertext: str) -> str:
+    """Decrypt a secret, returning "" if it cannot be read.
+
+    An unreadable value almost always means LEMBAS_SECRET_KEY changed. Failing
+    soft keeps the admin UI usable so the key can simply be re-entered.
+    """
+    if not ciphertext:
+        return ""
+    try:
+        return _fernet().decrypt(ciphertext.encode("ascii")).decode("utf-8")
+    except (InvalidToken, ValueError):
+        log.warning("could not decrypt a stored secret; has LEMBAS_SECRET_KEY changed?")
+        return ""
+
+
+def mask(secret: str) -> str:
+    """Render a secret for display: never the whole thing, just enough to identify it."""
+    if not secret:
+        return ""
+    if len(secret) <= 8:
+        return "*" * len(secret)
+    return f"{secret[:3]}{'*' * 8}{secret[-4:]}"