docs: Add Rustdoc plan to PLAN.md

Co-authored-by: aider (openai/andrew/openrouter/sonoma-sky-alpha) <aider@aider.chat>

PLAN.md

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+# Rustdoc Plan
+
+## Files Missing or Incomplete Rustdoc
+
+This plan tracks public modules, structs, functions, and traits that lack comprehensive `///` documentation comments. Rustdoc should cover:
+
+- Module-level docs explaining purpose and usage.
+- Public struct/enum/trait docs with fields/variants.
+- Public function/method docs with params, returns, and examples.
+- Error handling and edge cases where relevant.
+
+### High Priority (Core Public API)
+- `src/services/item_service.rs`: Missing docs for `ItemService` methods like `get_item`, `save_item`. Add examples for usage.
+- `src/services/async_item_service.rs`: `AsyncItemService` needs async-specific docs, including blocking task explanations.
+- `src/modes/server/common.rs`: `AppState`, `ApiResponse<T>`, and query structs lack full param/return docs.
+- `src/compression_engine/mod.rs`: `CompressionEngine` trait needs comprehensive docs for all methods.
+- `src/filter_plugin/mod.rs`: `FilterPlugin` trait and `FilterChain` need usage examples.
+
+### Medium Priority (Utilities and Helpers)
+- `src/common/status.rs`: `StatusInfo` and related structs need schema docs.
+- `src/db.rs`: All public functions like `open`, `insert_item` need error context and examples.
+- `src/config.rs`: `Settings` and sub-structs need deserialization notes.
+- `src/meta_plugin/mod.rs`: `MetaPlugin` trait methods need more examples; `process_metadata_outputs` lacks cases.
+
+### Low Priority (Internal/Helper)
+- `src/services/compression_service.rs`: `CompressionService` methods are simple but need brief docs.
+- `src/services/filter_service.rs`: `FilterService` needs plugin registration docs.
+- `src/modes/common.rs`: Utility functions like `format_size` need inline docs.
+
+## Generation Plan
+1. Run `cargo doc --no-deps` to generate and inspect current docs.
+2. Add `///` comments to missing items.
+3. Use `cargo doc --open` to verify.
+4. Ensure all public items are documented before release.
+
+## Commands
+```bash
+cargo doc --no-deps  # Generate docs without deps
+cargo doc --open     # Open generated docs in browser
+```