Models know how much context they hold

A column rather than a key in capabilities_json, which is rebuilt wholesale
from the submitted checkboxes on every save and would destroy a number
living in it.

0 means unknown, and unknown has to stay tellable from small: the context
percentage and automatic compaction both refuse to act on a figure nobody
supplied. Filled in from /v1/models where the runner advertises it --
OpenRouter, vLLM and llama.cpp each spell it differently, so context_from()
reads the four spellings actually in use, accepts a quoted number but not
"8192 tokens", and rejects anything outside 256..10,000,000. Applied on
discovery only when nothing is set: a refresh must never undo a correction,
since an administrator sets this precisely because the endpoint was wrong.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/lembas/web/templates/admin/model_detail.html

@@ -92,6 +92,18 @@
       </div>
     </div>
 
+    <div class="field">
+      <label class="field__label" for="context-length">Context length</label>
+      <input class="input" id="context-length" name="context_length" type="number"
+             min="0" step="1" placeholder="unknown"
+             value="{{ model.context_length or '' }}">
+      <p class="field__hint">
+        How many tokens this model can hold, filled in from the endpoint where
+        it says. Leave it empty if you do not know: the context percentage and
+        automatic compaction both stay off rather than working from a guess.
+      </p>
+    </div>
+
     <div class="field">
       <label class="field__label" for="description">Description</label>
       <textarea class="textarea" id="description" name="description" rows="2"