Add dynamic Business Metrics with sample e-commerce definitions
Replace hardcoded Keboola-specific metrics card in Data Catalog with
dynamic Jinja template that renders whatever metric YAMLs exist in
docs/metrics/. Add 10 sample e-commerce metric definitions across
4 categories (revenue, customers, marketing, support) that align
with the sample data generator tables.
Key changes:
- MetricParser: new category colors + dynamic sql_* field discovery
- _load_metrics_data(): scans docs/metrics/*/*.yml with prod fallback
- catalog.html: 240 lines hardcoded HTML -> 35 lines Jinja loop
- metric_modal.js: regex-based category class removal, new categories
- 21 tests validating YAML schema, parser, and loader
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