Increase Ollama context window to 16k tokens

Ollama defaults to 2048 tokens which is far too small for page context
data + 15 tool definitions + conversation history. Added num_ctx=16384
via the options field on all Ollama requests. With Qwen at ~4GB and
7GB VRAM free this fits comfortably.

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

src/Purrse.Api/Services/ChatService.cs

@@ -198,7 +198,8 @@ public class ChatService : IChatService
                 Model = modelName,
                 Messages = ollamaMessages,
                 Stream = false,
-                Tools = tools
+                Tools = tools,
+                Options = new OllamaRequestOptions { NumCtx = 16384 }
             };
 
             var response = await CallOllamaAsync(ollamaUrl, ollamaRequest);
@@ -270,7 +271,8 @@ public class ChatService : IChatService
                         Model = modelName,
                         Messages = ollamaMessages,
                         Stream = false,
-                        Tools = null // drop tools so the model focuses on the page data
+                        Tools = null, // drop tools so the model focuses on the page data
+                        Options = new OllamaRequestOptions { NumCtx = 16384 }
                     };
                     var retryResponse = await CallOllamaAsync(ollamaUrl, retryRequest);
                     rawContent = retryResponse?.Message?.Content;

src/Purrse.Core/DTOs/ChatDtos.cs

@@ -45,6 +45,16 @@ public class OllamaToolChatRequest
 
     [JsonPropertyName("tools")]
     public List<OllamaTool>? Tools { get; set; }
+
+    [JsonPropertyName("options")]
+    [JsonIgnore(Condition = JsonIgnoreCondition.WhenWritingNull)]
+    public OllamaRequestOptions? Options { get; set; }
+}
+
+public class OllamaRequestOptions
+{
+    [JsonPropertyName("num_ctx")]
+    public int NumCtx { get; set; }
 }
 
 public class OllamaToolMessage