Every error response in the { error, code, details? } format (step 2)

Moves every remaining hand-built error response onto ApiError: ~300
sites across faces, image, video, files, memories, versions,
duplicates, favorites, libraries, auth, clip/unified search, prefs,
reels, tts, insight chat, persona chat, knowledge and personas.

- 500s: the body was the error's text (anyhow / diesel / {:?}); now
  ApiError::internal logs it and sends a generic message.
- Each 4xx/502/503 has a stable code and a message written for users;
  statuses are unchanged. Shared helpers per module (faces_unavailable,
  person_not_found, unknown_library, insight_not_found, ...).
- Pre-format top-level fields kept where clients read them: upload 409
  duplicate_of / content_hash / library_name.
- PersonaChatError converts into ApiError in one place.
- Multipart upload errors use the format (error::multipart_config).
- Guard test: no non-test code builds HttpResponse::<error>() by hand.

Closes #202

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/data/mod.rs

@@ -1,6 +1,7 @@
 use std::{fs, str::FromStr};
 
 use crate::database::models::ImageExif;
+use crate::error::ApiError;
 use anyhow::{Context, anyhow};
 
 use chrono::{DateTime, Utc};
@@ -89,7 +90,7 @@ impl FromRequest for Claims {
             .map_or_else(
                 |e| {
                     error!("{}", e);
-                    err(crate::error::ApiError::unauthorized(
+                    err(ApiError::unauthorized(
                         "unauthorized",
                         "Your session has expired. Sign in again.",
                     )