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/common/config.rs

@@ -211,6 +211,12 @@ pub struct StorageConfig {
     /// Maximum upload file size in bytes (default: 10 GB).
     /// Applied as a hard limit to WebDAV PUT and streaming uploads.
     pub max_upload_size: usize,
+    /// Directory for upload spool temp files. When `Some`, large uploads are
+    /// spooled here instead of the OS default temp dir (often tmpfs/RAM in
+    /// containers, where the spool's page-cache counts against the cgroup
+    /// memory limit and can trigger OOMKill on large files). Env:
+    /// `OXICLOUD_UPLOAD_TMPDIR`.
+    pub upload_temp_dir: Option<PathBuf>,
     /// Which blob storage backend to use (`local`, `s3`, or `azure`).
     pub backend: StorageBackendType,
     /// S3-compatible backend configuration (used when `backend == S3`).
@@ -348,6 +354,7 @@ impl Default for StorageConfig {
             parallel_threshold: 100 * 1024 * 1024, // 100 MB
             trash_retention_days: 30,              // 30 days
             max_upload_size: MAX_UPLOAD_SIZE,
+            upload_temp_dir: None,
             backend: StorageBackendType::Local,
             s3: None,
             azure: None,
@@ -1212,6 +1219,14 @@ impl AppConfig {
             config.storage.max_upload_size = val;
         }
 
+        // Upload spool directory — keep large upload temp files off tmpfs/RAM
+        // (otherwise their page-cache counts against the cgroup memory limit).
+        if let Ok(dir) = env::var("OXICLOUD_UPLOAD_TMPDIR")
+            && !dir.trim().is_empty()
+        {
+            config.storage.upload_temp_dir = Some(PathBuf::from(dir.trim()));
+        }
+
         // Storage backend selection
         if let Ok(backend) = env::var("OXICLOUD_STORAGE_BACKEND") {
             match backend.to_lowercase().as_str() {