Add --file ingestion with restart-safe tailing (ticket 020)

ingest::run_file tails a path like tail -f: reads existing content,
follows appended lines, and detects file recreation/truncation (via
inode on unix, or a shrinking size) to resume tailing the new
instance from its start instead of exiting or stalling. Sequence
numbers keep incrementing across a recreation event since they're
ottyl's own line counter, not tied to the source file. stdin ingest
is unchanged and remains the default when --file is omitted.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

src/main.rs

@@ -38,6 +38,12 @@ struct Args {
     /// (e.g. `~/.config/ottyl/settings.json` on Linux).
     #[arg(long)]
     settings_path: Option<PathBuf>,
+
+    /// File to tail for log lines, instead of reading from stdin. Behaves like
+    /// `tail -f`: reads existing content then keeps following appended lines,
+    /// and keeps tailing if the file is deleted/recreated at the same path.
+    #[arg(long)]
+    file: Option<PathBuf>,
 }
 
 #[tokio::main]
@@ -86,7 +92,14 @@ async fn main() {
         }
     }
 
-    tokio::spawn(ingest::run(engine_tx.clone()));
+    match args.file.clone() {
+        Some(path) => {
+            tokio::spawn(ingest::run_file(engine_tx.clone(), path));
+        }
+        None => {
+            tokio::spawn(ingest::run(engine_tx.clone()));
+        }
+    }
 
     let app = api::router(engine_tx, &args.static_dir, settings, settings_path);
     let listener = TcpListener::bind(("0.0.0.0", args.port))
@@ -97,9 +110,13 @@ async fn main() {
         .expect("failed to read back bound address")
         .port();
 
+    let source = match &args.file {
+        Some(path) => format!("tailing {}", path.display()),
+        None => "reading logs from stdin".to_string(),
+    };
     eprintln!(
-        "ottyl serving on http://localhost:{} (reading logs from stdin)",
-        bound_port
+        "ottyl serving on http://localhost:{} ({})",
+        bound_port, source
     );
 
     axum::serve(listener, app)