feat: add foundation layer (Go module, config, database, Docker)

Squashed commits from foundation-layer branch:

- Initialize Go module and project skeleton (cmd/naviwatcher/main.go)
- Add configuration management with YAML parsing and validation
- Add database layer with schema migrations (artist_settings, external_releases, notifications_sent)
- Add CRUD operations for artist_settings, external_releases, notifications_sent
- Add Docker setup with multi-stage build and docker-compose
- Verify acceptance criteria (tests, vet, fmt)
- Update README.md with build/run/test instructions
- Fix: filter ignored releases in GetUnnotifiedReleases (spec compliance)
- Fix: add FK constraint on notifications_sent.rgid
- Fix: add config.yaml to .gitignore (security)
- Fix: run Docker container as non-root user
- Fix: pin alpine:3.21 instead of alpine:latest
- Fix: wrap migrations in transactions for atomicity

All 49 tests pass, go vet clean, Docker image builds successfully.

internal/database/artist_settings.go

@@ -0,0 +1,116 @@
+package database
+
+import (
+	"fmt"
+)
+
+// GetArtistSettings retrieves an artist_settings row by ID.
+// Returns sql.ErrNoRows if the artist is not found.
+func GetArtistSettings(db *DB, id string) (*ArtistSettings, error) {
+	var s ArtistSettings
+	err := db.Conn().QueryRow(
+		"SELECT id, name, ignore_singles, ignore_compilations, monitored FROM artist_settings WHERE id = ?",
+		id,
+	).Scan(&s.ID, &s.Name, &s.IgnoreSingles, &s.IgnoreCompilations, &s.Monitored)
+	if err != nil {
+		return nil, err
+	}
+	return &s, nil
+}
+
+// SaveArtistSettings inserts or replaces an artist_settings row.
+func SaveArtistSettings(db *DB, settings *ArtistSettings) error {
+	_, err := db.Conn().Exec(
+		"INSERT OR REPLACE INTO artist_settings (id, name, ignore_singles, ignore_compilations, monitored) VALUES (?, ?, ?, ?, ?)",
+		settings.ID, settings.Name, settings.IgnoreSingles, settings.IgnoreCompilations, settings.Monitored,
+	)
+	if err != nil {
+		return fmt.Errorf("save artist settings: %w", err)
+	}
+	return nil
+}
+
+// GetAllArtistSettings returns all rows from artist_settings.
+func GetAllArtistSettings(db *DB) ([]ArtistSettings, error) {
+	rows, err := db.Conn().Query(
+		"SELECT id, name, ignore_singles, ignore_compilations, monitored FROM artist_settings",
+	)
+	if err != nil {
+		return nil, fmt.Errorf("query all artist settings: %w", err)
+	}
+	defer rows.Close()
+
+	var results []ArtistSettings
+	for rows.Next() {
+		var s ArtistSettings
+		if err := rows.Scan(&s.ID, &s.Name, &s.IgnoreSingles, &s.IgnoreCompilations, &s.Monitored); err != nil {
+			return nil, fmt.Errorf("scan artist settings: %w", err)
+		}
+		results = append(results, s)
+	}
+	if err := rows.Err(); err != nil {
+		return nil, fmt.Errorf("iterate artist settings: %w", err)
+	}
+	return results, nil
+}
+
+// UpdateArtistSettings updates specific fields of an artist_settings row by ID.
+// The updates map keys must match column names: "name", "ignore_singles", "ignore_compilations", "monitored".
+func UpdateArtistSettings(db *DB, id string, updates map[string]interface{}) error {
+	if len(updates) == 0 {
+		return fmt.Errorf("no updates provided")
+	}
+
+	// Build the query using a fixed set of allowed columns to avoid dynamic SQL.
+	const baseQuery = "UPDATE artist_settings SET"
+
+	var args []interface{}
+	setClause := ""
+	for col, val := range updates {
+		switch col {
+		case "name":
+			if setClause != "" {
+				setClause += ", "
+			}
+			setClause += "name = ?"
+			args = append(args, val)
+		case "ignore_singles":
+			if setClause != "" {
+				setClause += ", "
+			}
+			setClause += "ignore_singles = ?"
+			args = append(args, val)
+		case "ignore_compilations":
+			if setClause != "" {
+				setClause += ", "
+			}
+			setClause += "ignore_compilations = ?"
+			args = append(args, val)
+		case "monitored":
+			if setClause != "" {
+				setClause += ", "
+			}
+			setClause += "monitored = ?"
+			args = append(args, val)
+		default:
+			return fmt.Errorf("unknown column: %s", col)
+		}
+	}
+
+	args = append(args, id)
+	query := fmt.Sprintf("%s %s WHERE id = ?", baseQuery, setClause)
+	result, err := db.Conn().Exec(query, args...)
+	if err != nil {
+		return fmt.Errorf("update artist settings: %w", err)
+	}
+
+	rowsAffected, err := result.RowsAffected()
+	if err != nil {
+		return fmt.Errorf("rows affected: %w", err)
+	}
+	if rowsAffected == 0 {
+		return fmt.Errorf("artist not found: %s", id)
+	}
+
+	return nil
+}