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/api/preferences.py

@@ -37,6 +37,35 @@ async def set_theme(db: Db, user: RequiredUser, theme: str = Body(..., embed=Tru
     return {"ok": True, "theme": theme}
 
 
+@router.post("/default-model")
+async def set_default_model(
+    db: Db, user: RequiredUser, model_id: str = Form("")
+) -> Response:
+    """Choose which model new chats start with.
+
+    An empty value clears the choice and falls back to the instance default.
+    Validated against what this user can actually reach, so a model they lose
+    access to cannot linger as a preference that silently fails later.
+    """
+    from lembas.security import permissions
+
+    model_id = model_id.strip()
+    if model_id and not permissions.can_use_model(db, user, model_id):
+        return RedirectResponse(
+            "/settings?error=That+model+is+not+available+to+you.", status_code=303
+        )
+
+    settings_map = {**(user.settings_json or {})}
+    if model_id:
+        settings_map["default_model"] = model_id
+    else:
+        settings_map.pop("default_model", None)
+    user.settings_json = settings_map
+    db.commit()
+
+    return RedirectResponse("/settings?saved=Default+model+updated.", status_code=303)
+
+
 @router.post("/password")
 async def change_password(
     request: Request,