refactor: rename size to uncompressed_size, add compressed_size and closed columns

Schema changes:
- Rename items.size to items.uncompressed_size for clarity
- Add compressed_size (INTEGER NULL) - tracks compressed file size on disk
- Add closed (BOOLEAN NOT NULL DEFAULT 1) - tracks whether item is fully written
- Existing items default to closed=true via migration

Lifecycle:
- Items created with closed=false, set to true on successful save/import
- Compressed size captured via fs::metadata() after compression writer closes
- Truncated uploads (413) get compressed_size set, closed=true, uncompressed_size=None
- Update command now backfills both uncompressed_size and compressed_size

Also includes bug fixes and dedup from prior review:
- Fix stream_raw_content_response using uncompressed_size for raw byte Content-Length
- ApiResponse::ok()/empty() constructors, TryFrom<ItemWithMeta> for ItemInfo
- tag_names() method on ItemWithMeta, meta_filter() on Settings
- Fix .unwrap() panics in compression engine Read/Write impls
- Fix TOCTOU race in stream_raw_content_response (now uses compressed_size)
- Fix swallowed write errors in meta plugins (digest, magic_file, exec)
- Fix term::stderr().unwrap() panic in item_service
- Deduplicate ItemService::new() calls across 20 API handlers
- ImportMeta supports #[serde(alias = "size")] for backward compat

All 75 tests, 67 doc tests pass. Clippy clean.

src/modes/common.rs

@@ -643,7 +643,7 @@ pub fn sanitize_tags(tags: &[String]) -> String {
 pub struct ExportMeta {
     pub ts: DateTime<Utc>,
     pub compression: String,
-    pub size: Option<i64>,
+    pub uncompressed_size: Option<i64>,
     pub tags: Vec<String>,
     pub metadata: HashMap<String, String>,
 }
@@ -653,8 +653,8 @@ pub struct ExportMeta {
 pub struct ImportMeta {
     pub ts: DateTime<Utc>,
     pub compression: String,
-    #[serde(default)]
-    pub size: Option<i64>,
+    #[serde(default, alias = "size")]
+    pub uncompressed_size: Option<i64>,
     #[serde(default)]
     pub tags: Vec<String>,
     #[serde(default)]