fix(upload): stream WebDAV/NextCloud PUT to disk to prevent OOM on large files

Large uploads (e.g. ~800 MB ISOs) could OOMKill the process, even on
dedup hits, due to three separate full-file-in-memory paths:

- NextCloud PUT (/remote.php/dav) buffered the entire body in RAM via
  body::to_bytes before any dedup logic, then re-wrote and re-hashed it.
  Now streams the body to a temp file with incremental BLAKE3 and goes
  through update_file_streaming (shared spool helper with the native
  WebDAV PUT handler); peak heap is ~one HTTP frame regardless of size.

- DedupService::store_chunks materialized every new chunk's data in a Vec
  before uploading. Now reads each new chunk by positioned I/O
  (read_exact_at, off the runtime via spawn_blocking) just before its
  upload; peak heap bounded to ~CHUNK_UPLOAD_CONCURRENCY x CDC_MAX_CHUNK.

- The upload spool used the OS temp dir, often tmpfs/RAM in containers
  where its page-cache counts against the cgroup memory limit. Add
  OXICLOUD_UPLOAD_TMPDIR to point the spool at real disk.

Also collapse a pre-existing clippy collapsible_else_if in carddav_handler.

Refs #404

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

src/interfaces/api/handlers/carddav_handler.rs

@@ -141,12 +141,10 @@ fn strip_username_prefix(path: &str) -> &str {
         } else {
             &path[pos + 1..]
         }
+    } else if uuid::Uuid::parse_str(path).is_ok() {
+        path
     } else {
-        if uuid::Uuid::parse_str(path).is_ok() {
-            path
-        } else {
-            ""
-        }
+        ""
     }
 }