perf: round 5 — CalDAV cursor streaming, SPA interning gaps, NC href prefix, per-request micro-allocs

Seven benchmark-gated changes (benches/ROUND5.md; BEFORE/AFTER bench +
equivalence gate each, rollback rule as ROUND2-4 — two intermediate
CalDAV shapes measured worse and were themselves rolled back before
shipping):

- CalDAV whole-calendar responses (REPORT no-range/sync-collection,
  depth-1 collection PROPFIND, .ics GET): buffered double-residency →
  ONE window-ordered scan (MIN(start_time) OVER (PARTITION BY ical_uid))
  streamed through a PG cursor, pages cut at UID boundaries. TTFB
  23.3→11.0 ms (2.1x), peak heap 14.2→8.0 MiB at 4k events / 45→24 MiB
  at 12k, wall +9-15% (documented trade, ZIP-streaming class); both
  multistatus and ICS byte-identical to the buffered output. Rejected
  shapes kept in the doc: per-page GROUP-BY keyset (3-4x wall) and
  per-uid ANY hydration (~20 µs/index descent).
- SPA listing interning gaps: folder/recent/favorites resources handlers
  (and the WebDAV pseudo-root) called raw Arc::from per row for the
  closed display set ROUND3 interned — now intern_display/intern_mime,
  4→0 allocs/row, byte-identical Arc contents.
- NC PROPFIND child hrefs: username + parent path encoded once per
  request instead of per child (543→165 ns/row, 13→4 allocs); native
  WebDAV href drops its intermediate encode String.
- suggest enrichment: entity clone + field re-clones per keystroke row →
  consume + move (166.5→126.8 µs/200 rows, 20→7 allocs/row).
- list_readable_by returns the cache's Arc (246→128 ns warm hit, 4→0
  allocs) — deep Vec clone per DAV-selector request removed.
- CardDAV REPORT: borrowed props, reused href buffer, exact-size etag
  quoting (3.04→2.34 ms per 5k-contact getetag poll).
- Auth span records: user_id.to_string() per request ×3 →
  tracing::field::display.

Checks: cargo fmt, clippy --all-features --all-targets -D warnings,
cargo test --workspace (523 passed). Follow-ups (CardDAV streaming,
&[&str] id batches, ::text UUID casts A/B, share-landing join) recorded
in benches/ROUND5.md.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017aJu9ghvuT8WqC31ZEGTBA

src/interfaces/api/handlers/folder_handler.rs

@@ -8,7 +8,8 @@ use std::collections::HashMap;
 use std::sync::Arc;
 
 use crate::application::dtos::display_helpers::{
-    category_for, format_file_size, icon_class_for, icon_special_class_for,
+    category_for, format_file_size, icon_class_for, icon_special_class_for, intern_display,
+    intern_mime,
 };
 use crate::application::dtos::file_dto::FileDto;
 use crate::application::dtos::folder_dto::{
@@ -482,9 +483,9 @@ pub async fn list_folder_resources(
                             created_at: row.created_at.timestamp() as u64,
                             modified_at: row.modified_at.timestamp() as u64,
                             is_root: false,
-                            icon_class: Arc::from("fas fa-folder"),
-                            icon_special_class: Arc::from("folder-icon"),
-                            category: Arc::from("Folder"),
+                            icon_class: intern_display("fas fa-folder"),
+                            icon_special_class: intern_display("folder-icon"),
+                            category: intern_display("Folder"),
                             // §14 provenance not selected by the resources query.
                             created_by: None,
                             updated_by: None,
@@ -518,13 +519,15 @@ pub async fn list_folder_resources(
                             name: row.name.clone(),
                             path: String::new(),
                             size: size_bytes,
-                            mime_type: Arc::from(mime),
+                            mime_type: intern_mime(mime),
                             folder_id: row.parent_id.map(|u| u.to_string()),
                             created_at: row.created_at.timestamp() as u64,
                             modified_at: row.modified_at.timestamp() as u64,
-                            icon_class: Arc::from(icon_class_for(&row.name, mime)),
-                            icon_special_class: Arc::from(icon_special_class_for(&row.name, mime)),
-                            category: Arc::from(category_for(&row.name, mime)),
+                            icon_class: intern_display(icon_class_for(&row.name, mime)),
+                            icon_special_class: intern_display(icon_special_class_for(
+                                &row.name, mime,
+                            )),
+                            category: intern_display(category_for(&row.name, mime)),
                             size_formatted: format_file_size(size_bytes),
                             sort_date: None,
                             content_hash,