chore: code review cleanup — fixes, deps, docs

Fixed:
- CLI help typo: "metatdata" -> "metadata"
- Filter buffer OOM: check size before loading into memory

Changed:
- #[inline] on HTML escape helpers for hot path performance
- Replaced once_cell and lazy_static with std::sync::LazyLock
- Removed unused once_cell and lazy_static crate dependencies

Refactored:
- Added module-level doc to services/ module

Documentation:
- README.md: zstd is native not external, "none" -> "raw"
- DESIGN.md: current schema and meta plugins section
- CHANGELOG.md: Unreleased section populated

src/services/mod.rs

@@ -1,3 +1,8 @@
+/// Business logic services for the Keep application.
+///
+/// This module provides the core service layer that orchestrates item storage,
+/// compression, metadata collection, and filtering. Services are used by both
+/// local CLI modes and the HTTP server.
 pub mod compression_service;
 pub mod error;
 pub mod filter_service;