refactor: remove dead OptimizedFileContent::Mmap variant

OptimizedFileContent::Mmap was constructed nowhere — the documented "Tier 2:
memory-mapped I/O (10-100 MB)" path was never wired, so optimized_inner only
ever returns Bytes (<10 MB) or Stream (>=10 MB). The variant survived only as
an enum case plus two dead match arms in the file and share download handlers.

Remove the variant and its arms, and fix the now-misleading retrieval-service
tier docs (everything >=10 MB streams via CDC chunk reassembly with the
backend read-ahead; there is no mmap tier). Behaviour is unchanged — the
deleted arms were unreachable.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JG5yYZ9s868mJwqT2Qz7ez

src/application/ports/file_ports.rs

@@ -93,7 +93,7 @@ pub trait FileUploadUseCase: Send + Sync + 'static {
 /// Optimized file content returned by the retrieval service.
 ///
 /// The handler only needs to map each variant to the appropriate HTTP
-/// response; all caching / transcoding / mmap decisions happen in the
+/// response; all caching / transcoding decisions happen in the
 /// application layer.
 pub enum OptimizedFileContent {
     /// Small-file content (possibly transcoded / compressed) already in RAM.
@@ -102,9 +102,7 @@ pub enum OptimizedFileContent {
         mime_type: Arc<str>,
         was_transcoded: bool,
     },
-    /// Memory-mapped file (10–100 MB).
-    Mmap(Bytes),
-    /// Streaming download for very large files (≥100 MB).
+    /// Streaming download for everything above the in-RAM cache threshold.
     Stream(Pin<Box<dyn Stream<Item = Result<Bytes, std::io::Error>> + Send>>),
 }
 

src/application/services/file_retrieval_service.rs

@@ -26,8 +26,8 @@ const CACHE_THRESHOLD: u64 = 10 * 1024 * 1024;
 /// Implements a multi-tier download strategy:
 /// - Tier 0: Write-behind cache (just-uploaded files still in RAM)
 /// - Tier 1: Hot cache + optional WebP transcoding (<10 MB)
-/// - Tier 2: Memory-mapped I/O (10–100 MB)
-/// - Tier 3: Streaming (≥100 MB)
+/// - Tier 2: Streaming for everything ≥10 MB — CDC chunk reassembly with the
+///   backend's read-ahead (`read_prefetch`); no whole-file buffering.
 pub struct FileRetrievalService {
     file_read: Arc<FileBlobReadRepository>,
     content_cache: Option<Arc<FileContentCache>>,

src/interfaces/api/handlers/file_handler.rs

@@ -750,20 +750,6 @@ impl FileHandler {
                     data, mime_type, ..
                 } => Self::build_cached_response(data, &mime_type, &disposition, &etag)
                     .into_response(),
-                OptimizedFileContent::Mmap(mmap_data) => Response::builder()
-                    .status(StatusCode::OK)
-                    .header(header::CONTENT_TYPE, &*file_dto.mime_type)
-                    .header(header::CONTENT_DISPOSITION, &disposition)
-                    .header(header::CONTENT_LENGTH, mmap_data.len())
-                    .header(header::ETAG, &etag)
-                    .header(
-                        header::CACHE_CONTROL,
-                        "private, max-age=3600, must-revalidate",
-                    )
-                    .header(header::ACCEPT_RANGES, "bytes")
-                    .body(Body::from(mmap_data))
-                    .unwrap()
-                    .into_response(),
                 OptimizedFileContent::Stream(pinned_stream) => Response::builder()
                     .status(StatusCode::OK)
                     .header(header::CONTENT_TYPE, &*file_dto.mime_type)

src/interfaces/api/handlers/share_handler.rs

@@ -501,21 +501,6 @@ async fn serve_share_file(
                 .body(Body::from(data))
                 .unwrap()
                 .into_response(),
-            OptimizedFileContent::Mmap(mmap_data) => Response::builder()
-                .status(StatusCode::OK)
-                .header(header::CONTENT_TYPE, &*mime)
-                .header(header::CONTENT_DISPOSITION, &disposition)
-                .header(header::CONTENT_LENGTH, mmap_data.len())
-                .header(header::ACCEPT_RANGES, "bytes")
-                .header(header::ETAG, &etag)
-                .header(
-                    header::CACHE_CONTROL,
-                    "private, max-age=3600, must-revalidate",
-                )
-                .header(header::VARY, "Cookie, Range")
-                .body(Body::from(mmap_data))
-                .unwrap()
-                .into_response(),
             OptimizedFileContent::Stream(stream) => Response::builder()
                 .status(StatusCode::OK)
                 .header(header::CONTENT_TYPE, &*mime)