feat(search): tag-based page search surface + per-page tags & collections

Add the /search surface (Pages / Chapters / Mangas tabs) backed by
per-user page tags and per-page collections: schema (migration 0023),
backend endpoints for page tags/collections and tagged-page aggregations
(with the OCR text-search param reserved at 501), plus the frontend API
clients, library Page-tags tab, collection page sections, page context
menu / AddTagsSheet, and reader long-press wiring. Includes the
continuous-reader navigation fixes (?page=N handling, chapter-reset
timing, back-button pops history) and tag-normalization hardening
accumulated on the branch.

Bump version 0.60.2 -> 0.62.0.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

backend/src/error.rs

@@ -38,6 +38,16 @@ pub enum AppError {
         message: String,
         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.
+    #[error("not implemented: {code}")]
+    NotImplemented {
+        code: &'static str,
+        message: &'static str,
+    },
     #[error(transparent)]
     Database(#[from] sqlx::Error),
     #[error(transparent)]
@@ -64,6 +74,7 @@ impl AppError {
             AppError::ServiceUnavailable(_) => "service_unavailable",
             AppError::TooManyRequests { .. } => "too_many_requests",
             AppError::ValidationFailed { .. } => "validation_failed",
+            AppError::NotImplemented { code, .. } => code,
             AppError::Database(sqlx::Error::RowNotFound) => "not_found",
             AppError::Database(_) => "internal_error",
             AppError::Storage(StorageError::NotFound) => "not_found",
@@ -124,6 +135,11 @@ impl IntoResponse for AppError {
                 message.clone(),
                 Some(details.clone()),
             ),
+            AppError::NotImplemented { message, .. } => (
+                StatusCode::NOT_IMPLEMENTED,
+                (*message).to_string(),
+                None,
+            ),
             AppError::Database(sqlx::Error::RowNotFound) => {
                 (StatusCode::NOT_FOUND, "not found".to_string(), None)
             }
@@ -180,5 +196,15 @@ mod tests {
         assert_eq!(AppError::Storage(StorageError::NotFound).code(), "not_found");
         assert_eq!(AppError::Database(sqlx::Error::RowNotFound).code(), "not_found");
         assert_eq!(AppError::Other(anyhow::anyhow!("oops")).code(), "internal_error");
+        // NotImplemented carries the code through so each pending
+        // feature gets its own stable identifier on the wire.
+        assert_eq!(
+            AppError::NotImplemented {
+                code: "text_search_not_yet_supported",
+                message: "x"
+            }
+            .code(),
+            "text_search_not_yet_supported"
+        );
     }
 }