Add LegiScan legislation ingestion and analysis queries

Adds ingest_legiscan.py to pull all US state + federal bills (2016-2026)
from the LegiScan API into legiscan_sessions and legiscan_bills tables.
Bills are keyword-tagged across 8 research categories (data_center,
ratepayer_protection, large_load, grid_impact, tax_incentive, etc.).
Loads ~1.3M bills; ~60K tagged relevant. Adds query_legiscan_bills.sql
with pre-built analysis queries including state/DC joins. Updates
database-tables.md, README.md, and research-ideas.md accordingly.

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

research-ideas.md

@@ -566,4 +566,58 @@ If you're interested in collaborating on any of these research directions, pleas
 
 ---
 
+## Legislative Analysis (LegiScan Data)
+
+**Status**: Data loaded — 1.3M bills across all US states + federal, 2016–2026; ~60K tagged relevant.  
+**Tables**: `legiscan_sessions`, `legiscan_bills`  
+**Query file**: `query_legiscan_bills.sql`
+
+### Research Questions
+
+**1. Ratepayer Cost Shifting**  
+Do states with high data center density show more legislative activity on ratepayer protection?  
+- Join `legiscan_bills WHERE 'ratepayer_protection' = ANY(relevance_tags)` to `master_data_centers` counts by state  
+- Test correlation between DC concentration and # of ratepayer bills introduced/passed  
+- Compare outcomes: do high-DC states pass or fail more ratepayer protections?
+
+**2. Data Center Legislative Wave**  
+Is there a measurable increase in DC-specific legislation after 2022 (AI boom)?  
+- Trend `data_center` and `large_load` tagged bills by `year_start`  
+- Cross-reference with major AI facility announcements (2022–2025)
+
+**3. Tax Incentive Geography**  
+Which states enacted tax incentives that may have influenced DC location decisions?  
+- `tax_incentive` bills with `status IN (4,8)` (passed/chaptered)  
+- Overlay with `master_data_centers` growth by state over the same period  
+- Candidate for difference-in-differences analysis
+
+**4. Grid Interconnection Policy**  
+Do states with `grid_impact` legislation show different EIA capacity expansion patterns?  
+- Join relevant bills to `energy_eia_operating_generator_capacity_flat` by state  
+- Look for correlations between grid policy activity and nameplate MW additions
+
+**5. Siting Preemption vs. Local Control**  
+Are states passing bills to streamline or restrict local siting authority?  
+- Full-text search within `siting_permitting` bills for "preemption" vs. "local control"  
+- Map bill outcomes by state political environment (cross-ref RDH vote data)
+
+### Suggested Joins
+
+```sql
+-- States with DCs and legislative activity by topic
+SELECT
+    dc.state,
+    COUNT(DISTINCT dc.id)                                              AS data_centers,
+    COUNT(DISTINCT lb.bill_id) FILTER (WHERE 'data_center'        = ANY(relevance_tags)) AS dc_bills,
+    COUNT(DISTINCT lb.bill_id) FILTER (WHERE 'ratepayer_protection'= ANY(relevance_tags)) AS ratepayer_bills,
+    COUNT(DISTINCT lb.bill_id) FILTER (WHERE 'tax_incentive'      = ANY(relevance_tags)
+                                         AND lb.status IN (4,8))      AS tax_incentives_passed
+FROM master_data_centers dc
+LEFT JOIN legiscan_bills lb ON dc.state = lb.state AND lb.is_relevant
+GROUP BY dc.state
+ORDER BY data_centers DESC;
+```
+
+---
+
 **Last Updated**: May 2026