feat: add Navidrome client with Subsonic API sync

Add the Navidrome client module that connects to a Navidrome server via
the Subsonic API, fetches artist and album data, and syncs it into the
local SQLite database.

- Add go-subsonic dependency for Subsonic API communication
- Create internal/navidrome/client.go with NavidromeClient wrapper
  - NewClient constructor with token-based auth
  - Ping health check with HTTP status validation
  - GetArtists fetches all artists via getArtists endpoint
  - GetArtistAlbums fetches albums per artist via getArtist endpoint
- Create internal/navidrome/sync.go with sync orchestration
  - SyncArtists upserts artists into artist_settings table
  - SyncAlbums fetches and stores albums for monitored artists
- Add local_albums table (migration 003) with FK to artist_settings
- Add LocalAlbum CRUD operations in internal/database/local_albums.go
- Full test coverage: 19 tests across client and sync packages
- All tests pass, go vet and go fmt clean

internal/navidrome/client.go

@@ -0,0 +1,118 @@
+package navidrome
+
+import (
+	"encoding/xml"
+	"fmt"
+	"io"
+	"net/http"
+	"time"
+
+	"naviwatcher/internal/config"
+
+	"github.com/delucks/go-subsonic"
+)
+
+// ArtistInfo represents a simplified artist from the Subsonic API.
+type ArtistInfo struct {
+	ID   string
+	Name string
+}
+
+// AlbumInfo represents a simplified album from the Subsonic API.
+type AlbumInfo struct {
+	ID       string
+	Name     string
+	ArtistID string
+}
+
+// NavidromeClient wraps the go-subsonic Client with application-specific configuration.
+type NavidromeClient struct {
+	client *subsonic.Client
+}
+
+// NewClient creates a new NavidromeClient from the given configuration.
+// It authenticates with the server immediately, returning an error if auth fails.
+func NewClient(cfg config.NavidromeConfig) (*NavidromeClient, error) {
+	client := &subsonic.Client{
+		Client:     &http.Client{Timeout: 30 * time.Second},
+		BaseUrl:    cfg.URL,
+		User:       cfg.User,
+		ClientName: "naviwatcher",
+	}
+
+	if err := client.Authenticate(cfg.Password); err != nil {
+		return nil, fmt.Errorf("authenticate with navidrome: %w", err)
+	}
+
+	return &NavidromeClient{client: client}, nil
+}
+
+// Ping checks connectivity to the Navidrome server.
+// Returns nil if the server is reachable and responds with a valid Subsonic OK status.
+func (nc *NavidromeClient) Ping() error {
+	resp, err := nc.client.Request("GET", "ping", nil)
+	if err != nil {
+		return fmt.Errorf("navidrome server is unreachable: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return fmt.Errorf("navidrome server returned HTTP %d", resp.StatusCode)
+	}
+
+	// Check Subsonic application-level status: the server can return HTTP 200
+	// with status="failed" for auth errors or other issues.
+	body, err := io.ReadAll(io.LimitReader(resp.Body, 4096))
+	if err != nil {
+		return fmt.Errorf("read ping response: %w", err)
+	}
+	var parsed subsonic.Response
+	if err := xml.Unmarshal(body, &parsed); err != nil {
+		return fmt.Errorf("parse ping response XML: %w", err)
+	}
+	if parsed.Status != "ok" && parsed.Error != nil {
+		return fmt.Errorf("navidrome ping failed: code %d: %s",
+			parsed.Error.Code, parsed.Error.Message)
+	}
+	return nil
+}
+
+// GetArtists fetches all artists from the Navidrome server.
+// Returns a slice of ArtistInfo with ID and Name populated.
+func (nc *NavidromeClient) GetArtists() ([]ArtistInfo, error) {
+	artists, err := nc.client.GetArtists(nil)
+	if err != nil {
+		return nil, fmt.Errorf("get artists: %w", err)
+	}
+
+	var result []ArtistInfo
+	for _, index := range artists.Index {
+		for _, artist := range index.Artist {
+			result = append(result, ArtistInfo{
+				ID:   artist.ID,
+				Name: artist.Name,
+			})
+		}
+	}
+	return result, nil
+}
+
+// GetArtistAlbums fetches all albums for a given artist from the Navidrome server.
+// The artistID should be the Subsonic ID of the artist.
+// Returns a slice of AlbumInfo with ID, Name, and ArtistID populated.
+func (nc *NavidromeClient) GetArtistAlbums(artistID string) ([]AlbumInfo, error) {
+	artist, err := nc.client.GetArtist(artistID)
+	if err != nil {
+		return nil, fmt.Errorf("get artist %s: %w", artistID, err)
+	}
+
+	var result []AlbumInfo
+	for _, album := range artist.Album {
+		result = append(result, AlbumInfo{
+			ID:       album.ID,
+			Name:     album.Name,
+			ArtistID: album.ArtistID,
+		})
+	}
+	return result, nil
+}