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/config.rs

@@ -148,6 +148,7 @@ pub struct ServerConfig {
     pub password_hash: Option<String>,
     pub cert_file: Option<PathBuf>,
     pub key_file: Option<PathBuf>,
+    pub cors_origin: Option<String>,
 }
 
 #[derive(Debug, Clone, Deserialize, Serialize)]
@@ -502,6 +503,10 @@ impl Settings {
         self.server.as_ref().and_then(|s| s.key_file.clone())
     }
 
+    pub fn server_cors_origin(&self) -> Option<String> {
+        self.server.as_ref().and_then(|s| s.cors_origin.clone())
+    }
+
     pub fn compression(&self) -> Option<String> {
         self.compression_plugin.as_ref().map(|c| c.name.clone())
     }