Users, groups, permissions, model settings and reasoning display

Four features, plus the schema machinery they needed.

**Schema sync.** The first live instance had data in it, and create_all
only creates missing *tables* -- a new column silently never appeared.
db/migrations.py now diffs the declared models against the database and
ALTER TABLE ... ADD COLUMN for what is missing, deriving a backfill
default from the column type (SQLite refuses a NOT NULL column without
one, and a Python-side `default=dict` cannot be expressed in DDL).
Verified against a copy of the live database: eight changes applied, all
rows preserved, second run a no-op. Renames, drops and retypes are still
manual and say so.

**Permissions.** A flat set of named booleans: an instance baseline
widened by each group the user belongs to. A group grants and never
denies -- with denies, "why can this user not do X" cannot be answered
without simulating every group. Admins bypass entirely, because an admin
can grant it back to themselves in two clicks and pretending otherwise
is theatre. Model *access* is separate: public, or granted to groups.
The picker is not the boundary -- switching a chat to a model you cannot
reach is a 403.

**Model settings.** Ordering, pinned-first, an instance default and a
per-user default, display names, descriptions, capability flags, and
uploaded images. Images are stored and served locally rather than by
URL: a remote URL makes every page render a request to a third party.
Uploads are validated by magic number, not the declared content type,
and stored under a random name. Models with no image get a generated
initial whose hue is derived from the model id, so it is stable.

**Reasoning display.** Streams into its own collapsible block above the
answer, labelled "Thought for 14 seconds", collapsed once finished, and
never replayed as context on the next turn. Two sources: the
reasoning_content delta field, and <think> tags inline in content -- the
latter needs a streaming splitter because the tags arrive split across
chunks. Models emitting no reasoning show nothing, via a :has() rule
rather than JavaScript. Verified against qwen35-9b on llama-swap: 694
reasoning events, 52 answer tokens, cleanly separated.

Two bugs found and fixed while testing:

- A bare `Mapped[list]` relationship is treated by SQLAlchemy as a scalar
  and returns None instead of []. It needs the element type.
- FastAPI substitutes the default for an empty form value, so with
  `x: str | None = Form(None)` a submitted `x=` is indistinguishable from
  an absent field. That silently broke clearing a system prompt or a
  temperature. update_chat now reads the raw form and checks key presence.

143 tests, ruff clean.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/lembas/services/uploads.py

@@ -0,0 +1,106 @@
+"""Storing uploaded images.
+
+Only model avatars use this today. Files are written under the data directory
+and served back by a dedicated route, never from a URL supplied by a user --
+a remote image URL would turn every page render into a request to a third
+party, which is both a privacy leak and a way to make the UI depend on someone
+else's uptime.
+"""
+
+from __future__ import annotations
+
+import logging
+import secrets
+from pathlib import Path
+
+from lembas.config import settings
+
+log = logging.getLogger(__name__)
+
+# Raster and vector formats a browser will render inline. Deliberately narrow:
+# every entry here is something that cannot execute in an <img> tag.
+ALLOWED_TYPES: dict[str, str] = {
+    "image/png": ".png",
+    "image/jpeg": ".jpg",
+    "image/webp": ".webp",
+    "image/gif": ".gif",
+}
+
+MAX_BYTES = 2 * 1024 * 1024  # 2 MB; these are 64px avatars
+
+# Magic numbers, checked against the declared content type. A browser sniffs
+# content, so trusting the client's Content-Type alone would let a file claim
+# to be a PNG and be served as something else.
+_SIGNATURES: tuple[tuple[bytes, str], ...] = (
+    (b"\x89PNG\r\n\x1a\n", "image/png"),
+    (b"\xff\xd8\xff", "image/jpeg"),
+    (b"GIF87a", "image/gif"),
+    (b"GIF89a", "image/gif"),
+)
+
+
+class UploadError(Exception):
+    """A rejected upload, with a message fit to show the user."""
+
+
+def _detect(payload: bytes) -> str | None:
+    for signature, media_type in _SIGNATURES:
+        if payload.startswith(signature):
+            return media_type
+    # WEBP is "RIFF" + 4 size bytes + "WEBP".
+    if payload[:4] == b"RIFF" and payload[8:12] == b"WEBP":
+        return "image/webp"
+    return None
+
+
+def models_dir() -> Path:
+    path = settings.uploads_dir / "models"
+    path.mkdir(parents=True, exist_ok=True)
+    return path
+
+
+def save_model_image(payload: bytes, declared_type: str) -> str:
+    """Validate and store a model avatar. Returns the stored filename."""
+    if not payload:
+        raise UploadError("The file was empty.")
+    if len(payload) > MAX_BYTES:
+        raise UploadError(f"Images must be under {MAX_BYTES // (1024 * 1024)} MB.")
+
+    actual = _detect(payload)
+    if actual is None:
+        raise UploadError("That does not look like a PNG, JPEG, WEBP or GIF image.")
+    if declared_type and declared_type.split(";")[0].strip() != actual:
+        # Not fatal on its own, but worth knowing about.
+        log.info("upload declared %s but is actually %s", declared_type, actual)
+
+    # Random name rather than the client's: no path traversal, no collisions,
+    # and no leaking whatever the uploader called the file.
+    filename = f"{secrets.token_hex(16)}{ALLOWED_TYPES[actual]}"
+    (models_dir() / filename).write_bytes(payload)
+    return filename
+
+
+def model_image_path(filename: str) -> Path | None:
+    """Resolve a stored filename to a path, refusing anything outside the dir."""
+    if not filename or "/" in filename or "\\" in filename or filename.startswith("."):
+        return None
+    path = (models_dir() / filename).resolve()
+    try:
+        path.relative_to(models_dir().resolve())
+    except ValueError:
+        return None
+    return path if path.is_file() else None
+
+
+def delete_model_image(filename: str) -> None:
+    path = model_image_path(filename)
+    if path is not None:
+        path.unlink(missing_ok=True)
+
+
+def media_type_for(filename: str) -> str:
+    suffix = Path(filename).suffix.lower()
+    for media_type, extension in ALLOWED_TYPES.items():
+        if extension == suffix:
+            return media_type
+    return "application/octet-stream"