Add configurable date extraction from filenames

[michaelbeutler]

Allow users to configure a date extraction pattern that parses a date from the original filename. The extracted date overrides $y, $m, $d (and $l) built-in variables. Falls back to current date if no match.

Example Config

settings:
  date_extraction:
    source: filename
    pattern: "(?P<year>[0-9]{4})(?P<month>[0-9]{2})(?P<day>[0-9]{2})"
    # or: "(?P<day>[0-9]{2})\\.(?P<month>[0-9]{2})\\.(?P<year>[0-9]{4})"

Implementation

Schema change:

• File: crates/paporg/src/config/schema.rs
• Add DateExtractionConfig struct with source, pattern fields
• Add date_extraction: Option<DateExtractionConfig> to the settings/config struct

Variable engine change:

• File: crates/paporg/src/config/variables.rs
• In get_builtin_variables(): accept an optional override date
• New function extract_date_from_filename(filename: &str, pattern: &str) -> Option<NaiveDate> that parses named groups year, month, day
• If date is extracted, use it for $y, $m, $d, $l instead of Utc::now()

Pipeline change:

• File: crates/paporg/src/pipeline/runner.rs
• Before step_extract_variables(), attempt date extraction from filename
• Pass extracted date (or None) to get_builtin_variables()

Acceptance Criteria

• Configurable regex with named groups year, month, day
• Extracted date overrides $y, $m, $d, $l
• Falls back to current date when pattern doesn't match
• Invalid dates (e.g., month 13) are rejected gracefully
• Covered by unit tests with various date formats