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/llm/client.go

@@ -0,0 +1,82 @@
+package llm
+
+import (
+	"fmt"
+	"log/slog"
+	"net/http"
+	"time"
+)
+
+// Config holds the configuration for an LLM client.
+type Config struct {
+	// Key is the API key for authentication.
+	Key string `yaml:"key"`
+	// SystemMessage is prepended to all conversations.
+	SystemMessage string `yaml:"system_message"`
+	// Timeout is the maximum duration for a query (e.g., "5m").
+	// Defaults to 5 minutes if empty.
+	Timeout string `yaml:"timeout"`
+	// URL is the base URL of the OpenAI-compatible API endpoint.
+	URL string `yaml:"url"`
+}
+
+// Validate checks that required configuration values are present and valid.
+func (cfg Config) Validate() error {
+	if cfg.Timeout != "" {
+		if _, err := time.ParseDuration(cfg.Timeout); err != nil {
+			return fmt.Errorf("invalid timeout: %w", err)
+		}
+	}
+	if cfg.URL == "" {
+		return fmt.Errorf("missing URL")
+	}
+	return nil
+}
+
+// Client wraps an OpenAI-compatible client with tool execution support.
+type Client struct {
+	baseURL       string
+	httpc         *http.Client
+	key           string
+	log           *slog.Logger
+	registry      *Registry
+	systemMessage string
+	timeout       time.Duration
+	tools         []APITool
+}
+
+// NewClient creates a new LLM client with the provided configuration.
+// The registry is optional; if nil, tool calling is disabled.
+func NewClient(
+	cfg Config,
+	registry *Registry,
+	log *slog.Logger,
+) (*Client, error) {
+	if err := cfg.Validate(); err != nil {
+		return nil, fmt.Errorf("invalid config: %w", err)
+	}
+
+	timeout := 5 * time.Minute
+	if cfg.Timeout != "" {
+		d, err := time.ParseDuration(cfg.Timeout)
+		if err != nil {
+			return nil, fmt.Errorf("parse timeout: %v", err)
+		}
+		timeout = d
+	}
+
+	llm := &Client{
+		baseURL:       cfg.URL,
+		key:           cfg.Key,
+		log:           log,
+		systemMessage: cfg.SystemMessage,
+		registry:      registry,
+		httpc:         &http.Client{},
+		timeout:       timeout,
+	}
+
+	// Convert tools from registry.
+	llm.tools = registry.ToAPI()
+
+	return llm, nil
+}