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_tools.go

@@ -0,0 +1,61 @@
+package llm
+
+import (
+	"context"
+	"fmt"
+	"log/slog"
+	"strings"
+)
+
+// callTools executes each tool call in the message and returns the
+// resulting tool result messages.
+func (c *Client) callTools(ctx context.Context, msg Message) (
+	results []Message, err error,
+) {
+	for _, tc := range msg.ToolCalls {
+		c.log.InfoContext(
+			ctx,
+			"calling tool",
+			slog.String("name", tc.Function.Name),
+			slog.String("args", tc.Function.Arguments),
+		)
+
+		result, err := c.registry.Execute(
+			ctx, tc.Function.Name, tc.Function.Arguments,
+		)
+		if err != nil {
+			c.log.ErrorContext(
+				ctx,
+				"failed to call tool",
+				slog.Any("error", err),
+				slog.String("name", tc.Function.Name),
+			)
+			result = fmt.Sprintf(
+				`{"ok": false, "error": %q}`, err,
+			)
+		} else {
+			c.log.InfoContext(
+				ctx,
+				"called tool",
+				slog.String("name", tc.Function.Name),
+			)
+		}
+
+		// Content cannot be empty.
+		if strings.TrimSpace(result) == "" {
+			result = `{"ok": true, "result": null}`
+		}
+
+		toolResult := Message{
+			Role: RoleTool,
+			ContentParts: []ContentPart{{
+				Type: ContentTypeText, Text: result,
+			}},
+			Name:       tc.Function.Name,
+			ToolCallID: tc.ID,
+		}
+		results = append(results, toolResult)
+	}
+
+	return results, nil
+}