feat(config): add database configuration layer with backend detection

Add AppConfig loaded from .env via dotenvy. DATABASE_URL prefix
determines the backend (sqlite:// → SQLite, postgresql:// → PostgreSQL).
ConfigError via thiserror gives clear messages on missing or unknown URLs.
Server logs the chosen backend at startup.

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

Cargo.toml

@@ -19,6 +19,7 @@ ssr = [
     "dep:tower-http",
     "dep:leptos_axum",
     "dep:tracing-subscriber",
+    "dep:dotenvy",
     "leptos/ssr",
     "leptos_meta/ssr",
     "leptos_router/ssr",
@@ -59,6 +60,8 @@ leptos_axum = { version = "0.7", optional = true }
 tower-http = { version = "0.5", features = ["fs"], optional = true }
 # Formateur de logs pour le terminal (affiche les messages tracing::info!...)
 tracing-subscriber = { version = "0.3", features = ["env-filter"], optional = true }
+# Charge automatiquement le fichier .env au démarrage du serveur
+dotenvy = { version = "0.15", optional = true }
 
 # --- Dépendances client uniquement (activées par la feature "hydrate") ---