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>

assets/logo-mark.svg

@@ -0,0 +1,49 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 64 64" width="64" height="64"
+     role="img" aria-label="LLeMbas">
+  <title>LLeMbas</title>
+  <desc>A silver mallorn leaf laid across a scored golden lembas wafer.</desc>
+  <defs>
+    <linearGradient id="m-wafer" x1="0" y1="0" x2="0.3" y2="1">
+      <stop offset="0" stop-color="#EACB74"/>
+      <stop offset="0.5" stop-color="#C9A227"/>
+      <stop offset="1" stop-color="#916F13"/>
+    </linearGradient>
+    <linearGradient id="m-leaf" x1="0.1" y1="1" x2="0.9" y2="0">
+      <stop offset="0" stop-color="#93A5B6"/>
+      <stop offset="0.4" stop-color="#F1F6FA"/>
+      <stop offset="1" stop-color="#B8C7D5"/>
+    </linearGradient>
+    <clipPath id="m-clip">
+      <rect x="6" y="6" width="52" height="52" rx="13"/>
+    </clipPath>
+  </defs>
+  <rect x="6" y="6" width="52" height="52" rx="13" fill="url(#m-wafer)"/>
+  <g clip-path="url(#m-clip)" fill="none" stroke-linecap="round">
+    <g stroke="#7A5C10" stroke-opacity="0.38" stroke-width="2">
+      <path d="M32 6 V58"/>
+      <path d="M6 32 H58"/>
+    </g>
+    <g stroke="#F6E3A8" stroke-opacity="0.3" stroke-width="1">
+      <path d="M33.2 6 V58"/>
+      <path d="M6 33.2 H58"/>
+    </g>
+  </g>
+  <rect x="7.1" y="7.1" width="49.8" height="49.8" rx="11.9"
+        fill="none" stroke="#7A5C10" stroke-opacity="0.3" stroke-width="1.2"/>
+  <g>
+    <path d="M21.2 44.8 L17 49.4" stroke="#8A9AA8" stroke-width="3"
+          stroke-linecap="round" fill="none"/>
+    <path d="M20.5 45.5 C13.8 31.7 23.8 20.9 45.5 18.5 C49.8 35 39.8 45.8 20.5 45.5 Z" fill="url(#m-leaf)"/>
+    <path d="M20.5 45.5 C28 38 36 29 45.5 18.5" fill="none" stroke="#61758A" stroke-opacity="0.5"
+          stroke-width="1.5" stroke-linecap="round"/>
+    <g fill="none" stroke="#61758A" stroke-opacity="0.32"
+       stroke-width="1" stroke-linecap="round">
+      <path d="M26.9 38.8 Q25.2 35.8 24.9 32.1"/>
+      <path d="M32.3 33.1 Q30.9 30.3 30.4 26.9"/>
+      <path d="M37.8 27.0 Q36.6 24.6 36.3 21.7"/>
+      <path d="M26.9 38.8 Q30.5 40.1 33.7 40.3"/>
+      <path d="M32.3 33.1 Q35.8 34.3 38.6 34.5"/>
+      <path d="M37.8 27.0 Q40.8 27.9 43.2 28.1"/>
+    </g>
+  </g>
+</svg>