Extend regex variable extraction: multiple named groups per pattern

[michaelbeutler]

Currently each ExtractedVariable defines one name and one pattern. To extract multiple values from a single regex (e.g., date + account ID from a Bank filename), support extracting all named capture groups from a single pattern.

Example Config

variables:
  extracted:
    - pattern: "(?P<file_year>[0-9]{4})(?P<file_month>[0-9]{2})(?P<file_day>[0-9]{2})_.*(?P<account_id>[0-9]{4}\\.[0-9]{4}\\.[0-9]{4})"
      source: filename

All named groups ($file_year, $file_month, $file_day, $account_id) become available as template variables.

Implementation

• File: crates/paporg/src/config/schema.rs

• Make name optional on ExtractedVariable — when omitted, all named capture groups become variables

• Add optional defaults: HashMap<String, String> for per-group defaults

• File: crates/paporg/src/config/variables.rs

• In extract_variables() (line ~36): when name is None, iterate over all named capture groups and insert each as a variable

• When name is Some, preserve current behavior (single variable extraction)

Acceptance Criteria

• Omitting name extracts all named capture groups as separate variables
• Providing name preserves existing single-variable behavior (backwards compatible)
• Per-group defaults work when a specific group doesn't match
• Transform applies to all extracted groups (or is skippable per group)
• Covered by unit tests