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/sessions.py

@@ -0,0 +1,99 @@
+"""Login session lifecycle.
+
+The browser holds an opaque random token in an httpOnly cookie. The database
+stores only its SHA-256, so a dump of the sessions table cannot be replayed as
+a live login. Tokens are compared by hash lookup, and revoking is a DELETE.
+"""
+
+from __future__ import annotations
+
+import hashlib
+import secrets
+from datetime import UTC, datetime, timedelta
+
+from sqlalchemy import select
+from sqlalchemy.orm import Session as DBSession
+
+from lembas.config import settings
+from lembas.db.models import Session as SessionRow
+from lembas.db.models import User
+
+COOKIE_NAME = "lembas_session"
+TOKEN_BYTES = 32
+
+
+def _hash_token(token: str) -> str:
+    return hashlib.sha256(token.encode("utf-8")).hexdigest()
+
+
+def create_session(
+    db: DBSession,
+    user: User,
+    *,
+    user_agent: str = "",
+    ip_address: str = "",
+) -> str:
+    """Open a session for a user and return the raw token for the cookie.
+
+    The raw token is returned exactly once and never persisted.
+    """
+    token = secrets.token_urlsafe(TOKEN_BYTES)
+    row = SessionRow(
+        user_id=user.id,
+        token_hash=_hash_token(token),
+        expires_at=datetime.now(UTC) + timedelta(seconds=settings.session_ttl),
+        user_agent=user_agent[:500],
+        ip_address=ip_address[:45],
+    )
+    db.add(row)
+    user.last_login_at = datetime.now(UTC)
+    db.commit()
+    return token
+
+
+def resolve_session(db: DBSession, token: str | None) -> User | None:
+    """Return the signed-in user for a cookie value, or None.
+
+    Expired and orphaned sessions are cleaned up as they are encountered, which
+    keeps the table tidy without needing a scheduled job.
+    """
+    if not token:
+        return None
+
+    row = db.scalar(select(SessionRow).where(SessionRow.token_hash == _hash_token(token)))
+    if row is None:
+        return None
+
+    # SQLite hands back naive datetimes even for timezone-aware columns.
+    expires_at = row.expires_at
+    if expires_at.tzinfo is None:
+        expires_at = expires_at.replace(tzinfo=UTC)
+
+    if expires_at < datetime.now(UTC):
+        db.delete(row)
+        db.commit()
+        return None
+
+    user = db.get(User, row.user_id)
+    if user is None or not user.active:
+        db.delete(row)
+        db.commit()
+        return None
+
+    return user
+
+
+def revoke_session(db: DBSession, token: str | None) -> None:
+    if not token:
+        return
+    row = db.scalar(select(SessionRow).where(SessionRow.token_hash == _hash_token(token)))
+    if row is not None:
+        db.delete(row)
+        db.commit()
+
+
+def revoke_all_for_user(db: DBSession, user: User) -> None:
+    """Sign a user out everywhere. Used when deactivating or changing a password."""
+    for row in db.scalars(select(SessionRow).where(SessionRow.user_id == user.id)):
+        db.delete(row)
+    db.commit()