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/security/passwords.py

@@ -0,0 +1,40 @@
+"""Password hashing.
+
+Argon2id via argon2-cffi, using the library's current recommended parameters.
+``needs_rehash`` lets stored hashes be upgraded transparently when those
+defaults tighten in a future release.
+"""
+
+from __future__ import annotations
+
+from argon2 import PasswordHasher
+from argon2.exceptions import InvalidHashError, VerifyMismatchError, VerificationError
+
+_hasher = PasswordHasher()
+
+MIN_PASSWORD_LENGTH = 8
+
+
+def hash_password(password: str) -> str:
+    return _hasher.hash(password)
+
+
+def verify_password(password: str, password_hash: str) -> bool:
+    try:
+        return _hasher.verify(password_hash, password)
+    except (VerifyMismatchError, VerificationError, InvalidHashError):
+        return False
+
+
+def needs_rehash(password_hash: str) -> bool:
+    try:
+        return _hasher.check_needs_rehash(password_hash)
+    except InvalidHashError:
+        return True
+
+
+def validate_password(password: str) -> str | None:
+    """Return a human-readable problem with the password, or None if it is fine."""
+    if len(password) < MIN_PASSWORD_LENGTH:
+        return f"Password must be at least {MIN_PASSWORD_LENGTH} characters."
+    return None