Add CFTC COT data integration with foundation data model layer

- New extraction package (cftc_cot): downloads yearly Disaggregated Futures ZIPs
  from CFTC, etag-based dedup, dynamic inner filename discovery, gzip normalization
- SQLMesh 3-layer architecture: raw (technical) → foundation (business model) → serving (mart)
- dim_commodity seed: conformed dimension mapping USDA ↔ CFTC codes — the commodity ontology
- fct_cot_positioning: typed, deduplicated weekly positioning facts for all commodities
- obt_cot_positioning: Coffee C mart with COT Index (26w/52w), WoW delta, OI ratios
- Analytics functions + REST API endpoints: /commodities/<code>/positioning[/latest]
- Dashboard widget: Managed Money net, COT Index card, dual-axis Chart.js chart
- 23 passing tests (10 unit + 2 SQLMesh model + existing regression suite)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

transform/sqlmesh_materia/models/foundation/dim_commodity.sql

@@ -0,0 +1,24 @@
+-- Commodity dimension: conforms identifiers across source systems.
+--
+-- This is the ontology seed. Each row is a commodity tracked by BeanFlows.
+-- As new sources are added (ICO, futures prices, satellite), their
+-- commodity identifiers are added as columns here — not as separate tables.
+-- As new commodities are added (cocoa, sugar), rows are added here.
+--
+-- References:
+--   usda_commodity_code → raw.psd_alldata.commodity_code
+--   cftc_commodity_code → raw.cot_disaggregated.cftc_commodity_code
+
+MODEL (
+  name foundation.dim_commodity,
+  kind SEED (
+    path '$root/seeds/dim_commodity.csv',
+    csv_settings (delimiter = ';')
+  ),
+  columns (
+    usda_commodity_code  varchar,
+    cftc_commodity_code  varchar,
+    commodity_name       varchar,
+    commodity_group      varchar
+  )
+);