chore: harden common errors, fastembed blocking, and ingest ownership

Run FastEmbed inference on spawn_blocking, propagate Surreal take
failures,
add AppError::internal and typed ingest/embedding parse errors, and take
owned file lists in ingestion payload construction.

common/src/error.rs

@@ -39,3 +39,11 @@ pub enum AppError {
     #[error("internal service error: {0}")]
     InternalError(String),
 }
+
+impl AppError {
+    /// Builds an [`AppError::InternalError`] from a displayable message.
+    #[must_use]
+    pub fn internal(msg: impl std::fmt::Display) -> Self {
+        Self::InternalError(msg.to_string())
+    }
+}