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.lock

@@ -315,6 +315,12 @@ dependencies = [
  "syn",
 ]
 
+[[package]]
+name = "dotenvy"
+version = "0.15.7"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "1aaf95b3e5c8f23aa320147307562d361db0ae0d51242340f558153b4eb2439b"
+
 [[package]]
 name = "drain_filter_polyfill"
 version = "0.1.3"
@@ -1590,6 +1596,7 @@ version = "0.1.0"
 dependencies = [
  "axum",
  "console_error_panic_hook",
+ "dotenvy",
  "leptos",
  "leptos_axum",
  "leptos_meta",