Make the codebase mypy-clean, drop dead media_utils and Pillow, rewrite README

src/core/transforms.py

@@ -6,6 +6,7 @@ value (e.g. rating 87 → tag "Hodnocení/87"), while the folder groups it
 """
 
 import re
+from collections.abc import Callable
 
 
 def rating_band(rating: int) -> str:
@@ -18,14 +19,14 @@ def rating_band(rating: int) -> str:
     return f"{low}–{high} %"
 
 
-def _decade_band(value) -> str:
+def _decade_band(value: str) -> str:
     """``decade_band`` transform: pull the leading integer and bucket it."""
     match = re.search(r"\d+", str(value))
     return rating_band(int(match.group())) if match else str(value)
 
 
 # Named value transforms usable from the tag schema (config-driven, extensible).
-TAG_TRANSFORMS = {
+TAG_TRANSFORMS: dict[str | None, Callable[[str], str]] = {
     None: str,
     "identity": str,
     "decade_band": _decade_band,