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/modes/info.rs

@@ -1,7 +1,7 @@
 use crate::config;
 use crate::modes::common::{OutputFormat, format_size};
 use crate::services::types::ItemWithMeta;
-use anyhow::{Result, anyhow};
+use anyhow::{Context, Result, anyhow};
 use clap::Command;
 use clap::error::ErrorKind;
 use serde::{Deserialize, Serialize};
@@ -141,7 +141,7 @@ fn show_item(
     }
 
     let item = item_with_meta.item;
-    let item_id = item.id.unwrap();
+    let item_id = item.id.context("Item missing ID")?;
     let item_tags: Vec<String> = item_with_meta.tags.iter().map(|t| t.name.clone()).collect();
 
     let mut table = crate::modes::common::create_table(false);
@@ -249,7 +249,7 @@ fn show_item_structured(
     let item_tags: Vec<String> = item_with_meta.tags.iter().map(|t| t.name.clone()).collect();
     let meta_map = item_with_meta.meta_as_map();
     let item = item_with_meta.item;
-    let item_id = item.id.unwrap();
+    let item_id = item.id.context("Item missing ID")?;
 
     let mut item_path_buf = data_path.clone();
     item_path_buf.push(item_id.to_string());