docs: add Databento integration plan and roadmap items

docs/roadmap/backlog/DATA-DB-003-databento-cache-management.yaml

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+id: DATA-DB-003
+title: Databento Cache Management
+status: backlog
+priority: medium
+dependencies:
+  - DATA-DB-001
+estimated_effort: 1 day
+created: 2026-03-28
+updated: 2026-03-28
+
+description: |
+  Implement cache lifecycle management for Databento data. Cache files should be
+  invalidated after configurable age (default 30 days) and when request parameters
+  change. Provide CLI tool for cache inspection and cleanup.
+
+acceptance_criteria:
+  - DatabentoCacheManager lists all cached entries
+  - Entries invalidated after max_age_days
+  - Parameters change detection triggers re-download
+  - Cache size tracking available
+  - CLI command to clear all cache
+  - CLI command to show cache statistics
+
+implementation_notes: |
+  Cache files stored in .cache/databento/:
+  - dbn_{hash}.parquet: Data file
+  - dbn_{hash}_meta.json: Metadata (download_date, params, rows)
+  
+  Cache invalidation rules:
+  1. Age > 30 days: re-download
+  2. Parameters changed: re-download
+  3. File corruption: re-download
+  
+  CLI commands:
+  - vault-dash cache list
+  - vault-dash cache clear
+  - vault-dash cache stats
+
+dependencies_detail:
+  - DATA-DB-001: Needs DatabentoCacheKey structure