fix(llm): normalize Ollama model tags to lowercase

Ollama's /api/tags returns whatever casing was used at pull time
(e.g. 'gemma3:12B' if the user ran 'ollama pull gemma3:12B'), but
/api/chat rejects mixed-case tags with a 400. The two code paths
are inconsistent, which surfaces the capitalized tag in the model
dropdown and then silently kills every chat request against it.

Lowercase on read (get_installed_models), on settings write
(update_settings_route), and on ensure_model() input so a legacy
mixed-case user setting can't trigger a spurious re-pull at
startup. The dropdown and stored settings are now always in the
form Ollama will actually accept.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/fabledassistant/services/llm.py

@@ -78,7 +78,14 @@ RAG_AUTO_SNIPPET = 4000
 
 
 async def get_installed_models() -> set[str]:
-    """Return set of installed Ollama model names (with and without :latest)."""
+    """Return set of installed Ollama model names (with and without :latest).
+
+    Tags are normalized to lowercase. Ollama stores whatever casing was used at
+    pull time (``ollama pull gemma3:12B`` keeps the ``B``), but inference calls
+    against the capitalized tag can 400 — the two code paths are inconsistent.
+    Lowercasing on read avoids exposing that mixed-case name in the UI and
+    keeps the validation check below from accepting a form Ollama will reject.
+    """
     try:
         async with httpx.AsyncClient(timeout=10.0) as client:
             resp = await client.get(f"{Config.OLLAMA_URL}/api/tags")
@@ -86,7 +93,7 @@ async def get_installed_models() -> set[str]:
             data = resp.json()
             names: set[str] = set()
             for m in data.get("models", []):
-                name = m["name"]
+                name = m["name"].lower()
                 names.add(name)
                 if name.endswith(":latest"):
                     names.add(name.removesuffix(":latest"))
@@ -98,6 +105,9 @@ async def get_installed_models() -> set[str]:
 
 async def ensure_model(model: str) -> None:
     """Check if model exists in Ollama, pull if missing."""
+    # Match the lowercase normalization in get_installed_models so a legacy
+    # mixed-case setting doesn't force a spurious re-pull at startup.
+    model = model.lower()
     try:
         installed = await get_installed_models()
         if model in installed or f"{model}:latest" in installed: