Add offline mosaic EXIF tagging (stitch --write-exif, tag_mosaic_exif CLI)

- spruce.exif: tag_mosaic_jpeg_for_scan_dir, resolve_machine_label_for_scan_dir; ProcessingSoftware for tile-stitched mosaics
- spruce.settings: load_config(require_credentials=False) for config without login
- scripts/tag_mosaic_exif.py and tests; stitch script --write-exif path

spruce/settings.py

@@ -99,13 +99,20 @@ def _clamp_workers(n: int) -> int:
 # ---------------------------------------------------------------------------
 
 
-def load_config(path: str) -> dict:
-    """Load and validate config.yaml. Exits on missing required fields."""
+def load_config(path: str, *, require_credentials: bool = True) -> dict:
+    """Load and validate config.yaml. Exits on missing required fields.
+
+    With ``require_credentials=False`` (e.g. offline EXIF tagging), username and
+    password are not required; only fields needed for tagging are defaulted.
+    """
     with open(path) as fh:
         cfg = yaml.safe_load(fh)
-    missing = [k for k in ("username", "password") if not cfg.get(k)]
-    if missing:
-        sys.exit(f"Config {path} is missing required fields: {missing}")
+    if cfg is None:
+        cfg = {}
+    if require_credentials:
+        missing = [k for k in ("username", "password") if not cfg.get(k)]
+        if missing:
+            sys.exit(f"Config {path} is missing required fields: {missing}")
     cfg.setdefault("base_url", "http://205.149.147.131:8010/")
     cfg.setdefault("image_base_url", "http://205.149.147.131:8011/")
     cfg.setdefault("output_dir", "archives")