Replace go-openai with native client

Drop the github.com/sashabaranov/go-openai dependency in favor of an
internal HTTP client.

The LLM package is now split into:
- llm.go: type definitions (Message, ChatRequest, ChatResponse, etc.)
- client.go: client construction and configuration
- client_completions.go: Completions and CompletionsStream methods
- client_models.go: ListModels
- client_tools.go: tool call execution

CompletionsStream uses typed SSE events (delta, done, message) so the
frontend can render progressive streaming output.

Move tool.go and tool_test.go from internal/tool/ into internal/llm/
to co-locate the registry with the client that consumes it.

Update job.go and service.go to use the new llm.Message types and
Completions/CompletionsStream methods. Remove the Voice field from
chat request/response (voice is now tracked via message metadata).

Frontend: handle delta/done/message SSE events, render a streaming
placeholder that receives progressive text deltas, and finalize with
full message binding (reasoning, tool calls, actions).

internal/service/static/main.css

@@ -554,6 +554,10 @@ body {
   color: var(--color-primary);
 }
 
+.message--streaming {
+  border-style: dashed;
+}
+
 .message__debug {
   display: none;
   padding: var(--s-2) var(--s-1);