feat: Add file scan and async hash system

Features:
1. scan command - Fast import without hash (skip_hash=true)
   - Scans directory structure
   - Generates deterministic UUIDs (SHA256(path|name|mac|mtime))
   - Stores full path in aliases.json
   - Inserts nodes in batches
   - Performance: 14243 nodes/sec (11857 files in 0.89s)

2. hash command - Async hash calculation
   - Multi-threaded (default: 4 threads)
   - Reads paths from aliases.json
   - Updates database with SHA256 hashes
   - Performance: 28 files/sec (11857 files in 417.58s)

Design:
- Import first, hash later (user can view tree immediately)
- Hash runs in background (non-blocking)
- Path stored in aliases.json (temporary solution)
- Deterministic UUIDs (same file = same UUID)

Performance breakdown:
- Scanning: 0.10s (11%)
- ID generation: 0.57s (64%)
- DB insertion: 0.21s (24%)
- Hash: 417.58s (async, background)

Files:
- src/scan.rs (new, 499 lines)
- src/main.rs (scan/hash commands)
- src/lib.rs (scan module)

Test result:
- warren user: 12658 nodes imported
- 11857 hashes calculated successfully

src/main.rs

@@ -28,6 +28,33 @@ enum Commands {
         #[command(subcommand)]
         action: ConfigCommands,
     },
+    /// Scan and import files from directory
+    Scan {
+        /// User ID
+        #[arg(short, long)]
+        user: String,
+        /// Directory to scan
+        #[arg(short, long)]
+        dir: String,
+        /// Batch size for database insertion
+        #[arg(short, long, default_value = "100")]
+        batch: usize,
+        /// Skip SHA256 hash calculation (faster import)
+        #[arg(short, long, default_value = "true")]
+        skip_hash: bool,
+        /// Number of threads for hash calculation (if skip_hash=false)
+        #[arg(short, long, default_value = "4")]
+        threads: usize,
+    },
+    /// Compute SHA256 hashes for imported files
+    Hash {
+        /// User ID
+        #[arg(short, long)]
+        user: String,
+        /// Number of threads for parallel hash calculation
+        #[arg(short, long, default_value = "4")]
+        threads: usize,
+    },
 }
 
 #[derive(Subcommand)]
@@ -73,6 +100,17 @@ async fn main() -> anyhow::Result<()> {
         Commands::Config { action } => {
             handle_config_command(action)?;
         }
+        Commands::Scan { user, dir, batch, skip_hash, threads } => {
+            use markbase::scan::ScanOptions;
+            let options = ScanOptions {
+                skip_hash,
+                threads,
+            };
+            markbase::scan::scan_directory(&user, &dir, batch, options)?;
+        }
+        Commands::Hash { user, threads } => {
+            markbase::scan::compute_hashes(&user, threads)?;
+        }
     }
     Ok(())
 }