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/compression_engine/program.rs

@@ -15,7 +15,13 @@ pub struct ProgramReader {
 
 impl Read for ProgramReader {
     fn read(&mut self, buf: &mut [u8]) -> std::io::Result<usize> {
-        self.stdout.as_mut().unwrap().read(buf)
+        match self.stdout.as_mut() {
+            Some(stdout) => stdout.read(buf),
+            None => Err(std::io::Error::new(
+                std::io::ErrorKind::BrokenPipe,
+                "stdout already taken",
+            )),
+        }
     }
 }
 
@@ -33,11 +39,23 @@ pub struct ProgramWriter {
 
 impl Write for ProgramWriter {
     fn write(&mut self, buf: &[u8]) -> std::io::Result<usize> {
-        self.stdin.as_mut().unwrap().write(buf)
+        match self.stdin.as_mut() {
+            Some(stdin) => stdin.write(buf),
+            None => Err(std::io::Error::new(
+                std::io::ErrorKind::BrokenPipe,
+                "stdin already taken",
+            )),
+        }
     }
 
     fn flush(&mut self) -> std::io::Result<()> {
-        self.stdin.as_mut().unwrap().flush()
+        match self.stdin.as_mut() {
+            Some(stdin) => stdin.flush(),
+            None => Err(std::io::Error::new(
+                std::io::ErrorKind::BrokenPipe,
+                "stdin already taken",
+            )),
+        }
     }
 }