Add -j/--workers flag and parallelize exiftool calls

Introduce -j/--workers CLI option defaulting to the number of CPUs. When
more than one worker is specified and not in dry-run mode, exiftool
invocations are dispatched concurrently via ThreadPoolExecutor and
tracked with as_completed, updating the progress bar as each completes.
Falls back to the original sequential/dry-run logic when a single worker
or dry-run is used.

src/emulsion/main.py

@@ -5,6 +5,7 @@ import subprocess
 import datetime
 import toml
 from alive_progress import alive_bar
+from concurrent.futures import ThreadPoolExecutor, as_completed
 
 CONFIG_PATH = os.path.expanduser("~/.config/emulsion/config.toml")
 
@@ -38,6 +39,7 @@ def parse_args(config):
 	parser.add_argument('--base-date', default=None, help='Base date or date/time (e.g. 2023-04-10 or 2023-04-10 12:00:00).')
 	parser.add_argument('--time-increment', type=int, default=None, help='Time increment in seconds between images.')
 	parser.add_argument('--dry-run', action='store_true', help='Show what would be changed without modifying files.')
+	parser.add_argument('-j', '--workers', type=int, default=os.cpu_count() or 1, help='Number of parallel workers to run exiftool; defaults to number of CPUs.')
 	parser.add_argument('--init-config', action='store_true', help='Create a default config file (if none exists) and exit.')
 
 	args = parser.parse_args()
@@ -236,6 +238,9 @@ def main():
 	try:
 		config = load_config()
 		args = parse_args(config)
+		# Default to number of CPUs if workers not specified
+		if args.workers is None:
+			args.workers = os.cpu_count() or 1
 
 		if args.init_config:
 			prompt_for_config(args)
@@ -261,41 +266,84 @@ def main():
 		print(f"Processing {total_files} file(s)...")
 
 		with alive_bar(total_files, title="Tagging files") as bar:
-			for f in files:
-				ext = os.path.splitext(f.lower())[1]
-				if ext not in ['.jpg', '.jpeg', '.tif', '.tiff']:
-					bar.text(f"Skipping unsupported file: {f}")
-					bar()
-					continue
+			if args.workers > 1 and not args.dry_run:
+				executor = ThreadPoolExecutor(max_workers=args.workers)
+				futures = {}
+				supported_idx = 0
+				for f in files:
+					ext = os.path.splitext(f)[1].lower()
+					if ext not in ['.jpg', '.jpeg', '.tif', '.tiff']:
+						bar.text(f"Skipping unsupported file: {f}")
+						bar()
+						continue
 
-				timestamp_str = current_dt.strftime("%Y:%m:%d %H:%M:%S")
-				cmd = build_exiftool_cmd(
-					file_path=f,
-					author=args.author,
-					lab=args.lab,
-					make=args.make,
-					model=args.model,
-					film=args.film,
-					timestamp=timestamp_str,
-					dry_run=args.dry_run
-				)
+					ts_dt = base_dt + datetime.timedelta(seconds=supported_idx * time_increment)
+					timestamp_str = ts_dt.strftime("%Y:%m:%d %H:%M:%S")
+					cmd = build_exiftool_cmd(
+						file_path=f,
+						author=args.author,
+						lab=args.lab,
+						make=args.make,
+						model=args.model,
+						film=args.film,
+						timestamp=timestamp_str,
+						dry_run=False
+					)
+					future = executor.submit(
+						subprocess.run, cmd, check=True,
+						stdout=subprocess.DEVNULL, stderr=subprocess.DEVNULL
+					)
+					futures[future] = (f, timestamp_str)
+					supported_idx += 1
 
-				if args.dry_run:
-					bar.text(f"DRY RUN: {cmd}")
-				else:
+				for future in as_completed(futures):
+					f, ts = futures[future]
 					try:
-						subprocess.run(
-							cmd,
-							check=True,
-							stdout=subprocess.DEVNULL,
-							stderr=subprocess.DEVNULL
-						)
-						bar.text(f"Updated {f} => {timestamp_str}")
+						future.result()
+						bar.text(f"Updated {f} => {ts}")
 					except subprocess.CalledProcessError as e:
 						bar.text(f"Failed to update {f}: {e}")
+					bar()
 
-				current_dt += datetime.timedelta(seconds=time_increment)
-				bar()
+				executor.shutdown(wait=True)
+			else:
+				supported_idx = 0
+				current_dt = base_dt
+				for f in files:
+					ext = os.path.splitext(f.lower())[1]
+					if ext not in ['.jpg', '.jpeg', '.tif', '.tiff']:
+						bar.text(f"Skipping unsupported file: {f}")
+						bar()
+						continue
+
+					timestamp_str = current_dt.strftime("%Y:%m:%d %H:%M:%S")
+					cmd = build_exiftool_cmd(
+						file_path=f,
+						author=args.author,
+						lab=args.lab,
+						make=args.make,
+						model=args.model,
+						film=args.film,
+						timestamp=timestamp_str,
+						dry_run=args.dry_run
+					)
+
+					if args.dry_run:
+						bar.text(f"DRY RUN: {cmd}")
+					else:
+						try:
+							subprocess.run(
+								cmd,
+								check=True,
+								stdout=subprocess.DEVNULL,
+								stderr=subprocess.DEVNULL
+							)
+							bar.text(f"Updated {f} => {timestamp_str}")
+						except subprocess.CalledProcessError as e:
+							bar.text(f"Failed to update {f}: {e}")
+
+					current_dt += datetime.timedelta(seconds=time_increment)
+					bar()
 
 	except KeyboardInterrupt:
 		print("\nInterrupted by user. Exiting.")