refactor: restructure internal/ into adapters/, infra/, and app layers

- internal/gemini/ → internal/adapters/openai/ (renamed package to openai)
- internal/pexels/ → internal/adapters/pexels/
- internal/config/   → internal/infra/config/
- internal/database/ → internal/infra/database/
- internal/locale/   → internal/infra/locale/
- internal/middleware/ → internal/infra/middleware/
- internal/server/   → internal/infra/server/

All import paths and call sites updated accordingly.

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

backend/internal/adapters/openai/client.go

@@ -0,0 +1,120 @@
+package openai
+
+import (
+	"bytes"
+	"context"
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"time"
+)
+
+const (
+	// openaiAPIURL is the OpenAI chat completions endpoint.
+	openaiAPIURL = "https://api.openai.com/v1/chat/completions"
+
+	// openaiModel is the default text generation model.
+	openaiModel = "gpt-4o-mini"
+
+	// openaiVisionModel supports image inputs.
+	openaiVisionModel = "gpt-4o"
+
+	maxRetries = 3
+)
+
+// Client is an HTTP client for the OpenAI API.
+type Client struct {
+	apiKey     string
+	httpClient *http.Client
+}
+
+// NewClient creates a new Client.
+func NewClient(apiKey string) *Client {
+	return &Client{
+		apiKey: apiKey,
+		httpClient: &http.Client{
+			Timeout: 90 * time.Second,
+		},
+	}
+}
+
+// generateContent sends text messages to the text model.
+func (c *Client) generateContent(ctx context.Context, messages []map[string]string) (string, error) {
+	return c.callOpenAI(ctx, openaiModel, 0.7, messages)
+}
+
+// generateVisionContent sends an image + text prompt to the vision model.
+// imageBase64 must be the raw base64-encoded image data (no data URI prefix).
+// mimeType defaults to "image/jpeg" if empty.
+func (c *Client) generateVisionContent(ctx context.Context, prompt, imageBase64, mimeType string) (string, error) {
+	if mimeType == "" {
+		mimeType = "image/jpeg"
+	}
+	dataURL := fmt.Sprintf("data:%s;base64,%s", mimeType, imageBase64)
+
+	messages := []any{
+		map[string]any{
+			"role": "user",
+			"content": []any{
+				map[string]any{
+					"type":      "image_url",
+					"image_url": map[string]string{"url": dataURL},
+				},
+				map[string]any{
+					"type": "text",
+					"text": prompt,
+				},
+			},
+		},
+	}
+	return c.callOpenAI(ctx, openaiVisionModel, 0.1, messages)
+}
+
+// callOpenAI is the shared HTTP transport for all OpenAI requests.
+// messages can be []map[string]string (text) or []any (vision with image content).
+func (c *Client) callOpenAI(ctx context.Context, model string, temperature float64, messages any) (string, error) {
+	body := map[string]any{
+		"model":       model,
+		"temperature": temperature,
+		"messages":    messages,
+	}
+
+	bodyBytes, err := json.Marshal(body)
+	if err != nil {
+		return "", fmt.Errorf("marshal request: %w", err)
+	}
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, openaiAPIURL, bytes.NewReader(bodyBytes))
+	if err != nil {
+		return "", fmt.Errorf("create request: %w", err)
+	}
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("Authorization", "Bearer "+c.apiKey)
+
+	resp, err := c.httpClient.Do(req)
+	if err != nil {
+		return "", fmt.Errorf("send request: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		raw, _ := io.ReadAll(resp.Body)
+		return "", fmt.Errorf("openai API error %d: %s", resp.StatusCode, string(raw))
+	}
+
+	var result struct {
+		Choices []struct {
+			Message struct {
+				Content string `json:"content"`
+			} `json:"message"`
+		} `json:"choices"`
+	}
+	if err := json.NewDecoder(resp.Body).Decode(&result); err != nil {
+		return "", fmt.Errorf("decode response: %w", err)
+	}
+	if len(result.Choices) == 0 {
+		return "", fmt.Errorf("empty response from OpenAI")
+	}
+	return result.Choices[0].Message.Content, nil
+}