feat(infra): add ServerState, MongoDB, auth middleware, and DaisyUI theme toggle

Introduce centralized ServerState (Arc-wrapped, Box::leaked configs) loaded
once at startup, replacing per-request dotenvy/env::var calls across all
server functions. Add MongoDB Database wrapper with connection pooling.
Add tower middleware that gates all /api/ server function endpoints behind
session authentication (401 for unauthenticated callers, except check-auth).
Fix DaisyUI theme toggle to use certifai-dark/certifai-light theme names
and replace hardcoded hex colors in main.css with CSS variables.

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

src/infrastructure/error.rs

@@ -1,22 +1,43 @@
 use axum::response::IntoResponse;
 use reqwest::StatusCode;
 
+/// Central error type for infrastructure-layer failures.
+///
+/// Each variant maps to an appropriate HTTP status code when converted
+/// into an Axum response.
 #[derive(thiserror::Error, Debug)]
 pub enum Error {
     #[error("{0}")]
     StateError(String),
 
+    #[error("database error: {0}")]
+    DatabaseError(String),
+
+    #[error("configuration error: {0}")]
+    ConfigError(String),
+
     #[error("IoError: {0}")]
     IoError(#[from] std::io::Error),
 }
 
+impl From<mongodb::error::Error> for Error {
+    fn from(err: mongodb::error::Error) -> Self {
+        Self::DatabaseError(err.to_string())
+    }
+}
+
 impl IntoResponse for Error {
     fn into_response(self) -> axum::response::Response {
         let msg = self.to_string();
         tracing::error!("Converting Error to Response: {msg}");
         match self {
-            Self::StateError(e) => (StatusCode::INTERNAL_SERVER_ERROR, e).into_response(),
-            _ => (StatusCode::INTERNAL_SERVER_ERROR, "Unknown error").into_response(),
+            Self::StateError(e) | Self::ConfigError(e) => {
+                (StatusCode::INTERNAL_SERVER_ERROR, e).into_response()
+            }
+            Self::DatabaseError(e) => (StatusCode::SERVICE_UNAVAILABLE, e).into_response(),
+            Self::IoError(_) => {
+                (StatusCode::INTERNAL_SERVER_ERROR, "Unknown error").into_response()
+            }
         }
     }
 }