Add OLLAMA_NUM_CTX config to reduce VRAM usage

Replaces the hardcoded num_ctx=32768 KV cache allocation with a
configurable env var defaulting to 8192. This significantly reduces
VRAM pressure when multiple services share the GPU.

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

src/fabledassistant/services/llm.py

@@ -80,7 +80,7 @@ async def stream_chat(
     options: dict | None = None,
 ) -> AsyncGenerator[str, None]:
     """Stream chat completion from Ollama, yielding content chunks."""
-    merged_options = {"num_ctx": 32768}
+    merged_options = {"num_ctx": Config.OLLAMA_NUM_CTX}
     if options:
         merged_options.update(options)
     payload: dict = {"model": model, "messages": messages, "stream": True, "options": merged_options}
@@ -126,7 +126,7 @@ async def stream_chat_with_tools(
     Thinking tokens are consumed by Ollama and not forwarded to the caller;
     only the final response content is yielded. Expect higher TTFT when enabled.
     """
-    options: dict = {"num_ctx": 32768}
+    options: dict = {"num_ctx": Config.OLLAMA_NUM_CTX}
     if tools:
         options["num_predict"] = 8192
     payload: dict = {