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).

vendor/modules.txt

@@ -8,11 +8,7 @@ github.com/google/uuid
 # github.com/robfig/cron/v3 v3.0.1
 ## explicit; go 1.12
 github.com/robfig/cron/v3
-# github.com/sashabaranov/go-openai v1.41.2
 ## explicit; go 1.18
-github.com/sashabaranov/go-openai
-github.com/sashabaranov/go-openai/internal
-github.com/sashabaranov/go-openai/jsonschema
 # golang.org/x/exp/typeparams v0.0.0-20231108232855-2478ac86f678
 ## explicit; go 1.18
 golang.org/x/exp/typeparams