config: simplify to single base_url with computed paths

Refactor configuration to use one source of truth for base URL

- Add GetBaseURL() to config package: queries system_config table first,
  falls back to BASE_URL env var
- Update SidecarHandler to accept config and use single base_url
- Compute opds/api paths from base_url instead of storing separately:
  - OPDS: base_url + /opds
  - API: base_url + /api
  - Device Sync: base_url + /api/sync
- Simplify OPDSHandler.getBaseURLs() to compute opds path
- Remove need for separate opds_base_url and api_base_url columns

Previously the system stored three separate URL config values that were
usually the same domain with different paths. Now store only base_url
and compute the paths, eliminating configuration redundancy.

internal/config/config.go

@@ -1,6 +1,7 @@
 package config
 
 import (
+	"context"
 	"fmt"
 	"os"
 	"strconv"
@@ -44,6 +45,30 @@ func (c *Config) DatabaseURL() string {
 		c.DatabaseUser, c.DatabasePassword, c.DatabaseHost, c.DatabasePort, c.DatabaseName)
 }
 
+// GetBaseURL returns the base URL from system configuration database with fallback to config/env var
+func GetBaseURL(ctx context.Context, db interface{}) string {
+	// Try to get from database first
+	type SystemConfigQuerier interface {
+		GetSystemConfig(ctx context.Context, key string) (SystemConfigRow, error)
+	}
+
+	if querier, ok := db.(SystemConfigQuerier); ok {
+		config, err := querier.GetSystemConfig(ctx, "base_url")
+		if err == nil && config.Value != "" {
+			return config.Value
+		}
+	}
+
+	// Fallback: return empty string - caller should use their own fallback
+	return ""
+}
+
+// SystemConfigRow represents a system configuration row
+type SystemConfigRow struct {
+	Key   string
+	Value string
+}
+
 func getEnv(key, defaultValue string) string {
 	if value := os.Getenv(key); value != "" {
 		return value