feat: filename template expansion with {date} placeholder

- Add expand_filename_template() to yaml_config.py
- Supports {author}, {from}, {to}, {date}, {ext} placeholders
- {date} formats as DD_MM_YYYY (e.g. 31_03_2026)
- Spaces in author replaced with underscores for safe filenames
- Unknown placeholders left as-is
- Add comprehensive CONFIG.md documentation covering YAML setup, migration, and template syntax

redmine_reporter/yaml_config.py

@@ -56,3 +56,46 @@ def check_file_permissions(path: Path) -> list[str]:
             f"Expected 0600. Fix with: chmod 600 {path}"
         )
     return warnings
+
+
+def expand_filename_template(
+    template: str,
+    *,
+    author: str = "",
+    from_date: str = "",
+    to_date: str = "",
+    ext: str = "",
+) -> str:
+    """Expand placeholders in a filename template.
+
+    Supported placeholders:
+      {author}  — author name
+      {from}    — start date (YYYY-MM-DD)
+      {to}      — end date (YYYY-MM-DD)
+      {date}    — end date formatted as DD_MM_YYYY
+      {ext}     — file extension without dot
+
+    Unknown placeholders are left as-is.
+    """
+    date_dd_mm_yyyy = ""
+    if to_date:
+        try:
+            from datetime import datetime
+
+            dt = datetime.strptime(to_date, "%Y-%m-%d")
+            date_dd_mm_yyyy = dt.strftime("%d_%m_%Y")
+        except ValueError:
+            date_dd_mm_yyyy = to_date
+
+    replacements = {
+        "author": author.replace(" ", "_"),
+        "from": from_date,
+        "to": to_date,
+        "date": date_dd_mm_yyyy,
+        "ext": ext,
+    }
+
+    result = template
+    for key, value in replacements.items():
+        result = result.replace("{" + key + "}", value)
+    return result