Add endowment spending distribution, move planning docs to private

- Add IPEDS F2H03C (spending distribution for current use) to endowment schema, loader, queries, and dashboard
- Endowment tab now shows spend rate alongside investment return rate
- Move planning docs to planning/ directory (gitignored)
- Update data dictionary

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/admin_analytics/db/schema.py

@@ -108,6 +108,7 @@ TABLES = {
             endowment_eoy BIGINT,
             new_gifts BIGINT,
             net_investment_return BIGINT,
+            spending_distribution BIGINT,
             other_changes BIGINT,
             total_private_gifts BIGINT,
             total_investment_return BIGINT,