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/mod.rs

@@ -2,6 +2,7 @@ pub mod api;
 pub mod errors;
 pub mod middleware;
 pub mod nextcloud;
+pub mod range_requests;
 pub mod upload_spool;
 pub mod web;