docs: mark CONV-001 and DATA-DB-003 as done

docs/roadmap/done/CONV-001-historical-gld-backing.yaml

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+id: CONV-001
+title: Historical GLD Backing Lookup
+status: backlog
+priority: P1
+size: S
+depends_on:
+  - PRICING-001
+tags: [conversion, historical]
+summary: Support lookups of GLD ounces-per-share for historical dates.
+acceptance_criteria:
+  - `gld_ounces_per_share(entry_date)` returns accurate backing for any date
+  - Formula validates against known points (launch 2004, current 2026)
+  - Conversion between GLD shares and gold oz uses entry_date backing
+  - Error handling for dates outside GLD lifetime (pre-2004)
+  - Cache-friendly for repeated lookups
+notes:
+  - GLD launched Nov 2004 with 0.10 oz/share
+  - Current backing ~0.0916 oz/share (2026)
+  - Formula: `0.10 × e^(-0.004 × years_since_2004)`
+  - Future enhancement: fetch actual NAV/ounce from spdrgoldshares.com
+implementation_hints:
+  - Already implemented for current date in `app/domain/instruments.py`
+  - Extend to accept `date` parameter
+  - Clamp dates to GLD lifetime (Nov 2004 to present)
+  - Add historical validation tests