feat(analysis): add ocrs OCR backend and OCR-driven page text search

Adds an in-process ocrs OCR backend as the active analysis engine
(vision left dormant), enables OCR text search on the page-tag
aggregation endpoints, and reshapes the admin analysis/settings UI to
present the OCR-only surface. Bumps version 0.89.0 -> 0.90.0.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/error.rs

@@ -39,10 +39,10 @@ pub enum AppError {
         details: serde_json::Value,
     },
     /// 501 — the wire shape is accepted but the feature isn't built yet.
-    /// Carries a `&'static str` snake_case code so clients can detect
-    /// the specific pending feature (`text_search_not_yet_supported`,
-    /// etc.) without parsing the message. Used today by the `?text=`
-    /// reservation on the page-tag aggregation endpoints.
+    /// Carries a `&'static str` snake_case code so clients can detect the
+    /// specific pending feature without parsing the message. Generic; kept
+    /// for future reservations (the `?text=` page-tag reservation that first
+    /// used it now performs real OCR search).
     #[error("not implemented: {code}")]
     NotImplemented {
         code: &'static str,