Split the model admin into a list and a page per model

/admin/models rendered a full edit form for every model. With eight that
was merely long; with a hundred it was unusable, which is the report.

The list is now compact rows only -- avatar, name, badges, position,
reorder buttons, Edit link -- with search across id and display name,
filter tabs (All / Enabled / Disabled / Pinned / Restricted, each with a
count), a connection filter, and pagination at 40. Filters are links, so
a filtered view is a real URL you can keep. Editing moved to
/admin/models/{id}/edit, one model per page, with Previous/Next links so
a freshly imported connection can be tidied without returning to the
list each time.

Measured with 128 models: the list is 73 KB showing 40 rows over 4
pages, and a detail page is 17 KB. The old page would have rendered all
128 forms into one response.

Reordering needed rethinking at that size too. Up/down is fine for
nudging a model one place but hopeless for moving it sixty, so the
detail page has a position field you type into; the value is clamped and
a non-numeric one is ignored rather than throwing. The move buttons take
a `back` field so they return to whatever filtered, paginated view they
were pressed on instead of dumping you at page 1.

Also adds a select-all checkbox for the bulk bar, scoped to a container
selector rather than the page so a future list can carry more than one.

212 tests, ruff clean.

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

README.md

@@ -45,8 +45,9 @@ runtime. Clone it, `pip install -e .`, run it.
   inside it
 - **OpenAI connections** — point at OpenAI, LM Studio, vLLM, llama.cpp,
   llama-swap, Ollama or OpenRouter; models are discovered and cached
-- **Model settings** — ordering, pinned models, an instance default and a
-  per-user default, custom names and descriptions, uploaded model images
+- **Model settings** — searchable, filterable list with a page per model:
+  ordering, pinned models, an instance default and a per-user default, custom
+  names, descriptions and images. Scales to hundreds of models
 - **Users, groups & permissions** — per-group grants that union rather than
   override, and model access restricted to chosen groups
 - **Accounts** — first account becomes the administrator, argon2 password