fix: harden security, eliminate panics, remove dead code, add Dockerfile

Security:
- Use constant-time password comparison (subtle crate) to prevent timing attacks
- Replace permissive CORS with configurable origin-restricted CORS
- Add TLS warning when password auth is used without HTTPS

Bug fixes:
- Convert MetaPlugin panics to anyhow::Result (get_meta_plugin, outputs_mut, options_mut)
- Replace item.id.unwrap() with proper error handling across 15 call sites
- Fix panic on unknown column type in list mode
- Fix conflicting PIPESIZE constant (was 8192 vs 65536, now unified to 8192)
- Add 256MB filter chain buffer limit to prevent OOM
- Gracefully skip unregistered plugins instead of panicking

Dead code removal:
- Delete unused filter parser files (filter_parser.rs, filter.pest, parser/ module)
- ~260 lines of dead PEG parser code removed

Code consolidation:
- Add is_content_binary_from_metadata() helper (was duplicated in 4 places)
- Simplify save_item_raw() to delegate to save_item_raw_streaming() (~90 lines removed)

Incomplete features:
- Populate filter_plugins in status output from global registry
- Add FallbackMagicFileMetaPlugin (was referenced but never implemented)
- Document init_plugins() as intentional no-op

Infrastructure:
- Add Dockerfile (static musl binary on scratch, 4.8MB)
- Add .dockerignore
- Add cors_origin to ServerConfig and config.rs

src/services/async_item_service.rs

@@ -155,11 +155,10 @@ impl AsyncItemService {
                     .map(|s| s.to_string())
                     .unwrap_or_else(|| "application/octet-stream".to_string());
 
-                let is_binary = if let Some(text_val) = metadata.get("text") {
-                    text_val == "false"
-                } else {
-                    crate::common::is_binary::is_binary(&content_clone)
-                };
+                let is_binary = crate::common::is_binary::is_content_binary_from_metadata(
+                    &metadata,
+                    &content_clone,
+                );
 
                 Ok::<_, CoreError>((mime_type, is_binary))
             })