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/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)