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/pexels/client.go

@@ -0,0 +1,77 @@
+package pexels
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"net/url"
+	"time"
+)
+
+const (
+	pexelsSearchURL    = "https://api.pexels.com/v1/search"
+	defaultPlaceholder = "https://images.pexels.com/photos/1640777/pexels-photo-1640777.jpeg?auto=compress&cs=tinysrgb&w=1260&h=750"
+)
+
+// PhotoSearcher can search for a photo by text query.
+type PhotoSearcher interface {
+	SearchPhoto(ctx context.Context, query string) (string, error)
+}
+
+// Client is an HTTP client for the Pexels Photos API.
+type Client struct {
+	apiKey     string
+	httpClient *http.Client
+}
+
+// NewClient creates a new Pexels client.
+func NewClient(apiKey string) *Client {
+	return &Client{
+		apiKey: apiKey,
+		httpClient: &http.Client{
+			Timeout: 10 * time.Second,
+		},
+	}
+}
+
+// SearchPhoto searches for a landscape photo matching query.
+// Returns a default placeholder URL if no photo is found or on error.
+func (c *Client) SearchPhoto(ctx context.Context, query string) (string, error) {
+	params := url.Values{}
+	params.Set("query", query)
+	params.Set("per_page", "1")
+	params.Set("orientation", "landscape")
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, pexelsSearchURL+"?"+params.Encode(), nil)
+	if err != nil {
+		return defaultPlaceholder, fmt.Errorf("create request: %w", err)
+	}
+	req.Header.Set("Authorization", c.apiKey)
+
+	resp, err := c.httpClient.Do(req)
+	if err != nil {
+		return defaultPlaceholder, fmt.Errorf("send request: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return defaultPlaceholder, fmt.Errorf("pexels API error: status %d", resp.StatusCode)
+	}
+
+	var result struct {
+		Photos []struct {
+			Src struct {
+				Medium string `json:"medium"`
+			} `json:"src"`
+		} `json:"photos"`
+	}
+	if err := json.NewDecoder(resp.Body).Decode(&result); err != nil {
+		return defaultPlaceholder, fmt.Errorf("decode response: %w", err)
+	}
+
+	if len(result.Photos) == 0 || result.Photos[0].Src.Medium == "" {
+		return defaultPlaceholder, nil
+	}
+	return result.Photos[0].Src.Medium, nil
+}