perf(dav): stream NC PROPFIND in batches; Range + 304 on WebDAV GETs

Two DAV-surface fixes that replicate patterns the codebase already had:

NC PROPFIND (folder case) previously loaded EVERY child via unbounded
list_files/list_folders and serialized the entire multistatus into one
Vec (~2 KB per entry — a 50k-file folder meant ~100 MB of buffer per
request, repeated constantly by sync clients). It now mirrors the native
WebDAV handler's streaming builder: children are fetched in pages of
PROPFIND_BATCH_SIZE (500), each page's favorites and oc:fileids are
resolved with two batch queries, and the XML is yielded chunk by chunk —
memory stays O(batch) and the first byte flows immediately. The
single-file PROPFIND keeps a small buffered variant; the multistatus
opening tag is factored into a shared helper so the namespace set cannot
diverge.

WebDAV GET (native and NC) ignored the Range header and never compared
the ETag it emitted, so mount-style clients (rclone, davfs2, Finder)
re-transferred whole files on every seek, resume, or revalidation. New
shared `interfaces::range_requests` helpers — same semantics as the REST
download endpoint, which now reuses the 304 helper too — give both GETs
If-None-Match → 304, Range → 206/416, and Accept-Ranges advertising.

https://claude.ai/code/session_01Dp3oWon5GBMVn4j3QXZdgx

src/interfaces/api/handlers/file_handler.rs

@@ -20,6 +20,7 @@ use crate::application::ports::{file_ports::OptimizedFileContent, folder_ports::
 use crate::common::di::AppState;
 use crate::interfaces::errors::AppError;
 use crate::interfaces::middleware::auth::AuthUser;
+use crate::interfaces::range_requests::not_modified_response;
 use crate::{application::dtos::file_dto::FileDto, domain::services::authorization::Permission};
 use std::sync::Arc;
 
@@ -600,16 +601,8 @@ impl FileHandler {
         let etag = format!("\"{}\"", file_dto.etag);
 
         // ── ETag (304 Not Modified) ──────────────────────────────────
-        if let Some(inm) = headers.get(header::IF_NONE_MATCH)
-            && let Ok(client_etag) = inm.to_str()
-            && (client_etag == etag || client_etag == "*")
-        {
-            return Response::builder()
-                .status(StatusCode::NOT_MODIFIED)
-                .header(header::ETAG, &etag)
-                .body(Body::empty())
-                .unwrap()
-                .into_response();
+        if let Some(resp) = not_modified_response(&headers, &etag) {
+            return resp.into_response();
         }
 
         // ── Range Requests ───────────────────────────────────────────