Initial import from garrytan/gstack@026751e (main snapshot via local relay)

Source: https://github.com/garrytan/gstack/commit/026751e

review/specialists/data-migration.md

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+# Data Migration Specialist Review Checklist
+
+Scope: When SCOPE_MIGRATIONS=true
+Output: JSON objects, one finding per line. Schema:
+{"severity":"CRITICAL|INFORMATIONAL","confidence":N,"path":"file","line":N,"category":"data-migration","summary":"...","fix":"...","fingerprint":"path:line:data-migration","specialist":"data-migration"}
+Optional: line, fix, fingerprint, evidence, test_stub.
+If no findings: output `NO FINDINGS` and nothing else.
+
+---
+
+## Categories
+
+### Reversibility
+- Can this migration be rolled back without data loss?
+- Is there a corresponding down/rollback migration?
+- Does the rollback actually undo the change or just no-op?
+- Would rolling back break the current application code?
+
+### Data Loss Risk
+- Dropping columns that still contain data (add deprecation period first)
+- Changing column types that truncate data (varchar(255) → varchar(50))
+- Removing tables without verifying no code references them
+- Renaming columns without updating all references (ORM, raw SQL, views)
+- NOT NULL constraints added to columns with existing NULL values (needs backfill first)
+
+### Lock Duration
+- ALTER TABLE on large tables without CONCURRENTLY (PostgreSQL)
+- Adding indexes without CONCURRENTLY on tables with >100K rows
+- Multiple ALTER TABLE statements that could be combined into one lock acquisition
+- Schema changes that acquire exclusive locks during peak traffic hours
+
+### Backfill Strategy
+- New NOT NULL columns without DEFAULT value (requires backfill before constraint)
+- New columns with computed defaults that need batch population
+- Missing backfill script or rake task for existing records
+- Backfill that updates all rows at once instead of batching (locks table)
+
+### Index Creation
+- CREATE INDEX without CONCURRENTLY on production tables
+- Duplicate indexes (new index covers same columns as existing one)
+- Missing indexes on new foreign key columns
+- Partial indexes where a full index would be more useful (or vice versa)
+
+### Multi-Phase Safety
+- Migrations that must be deployed in a specific order with application code
+- Schema changes that break the current running code (deploy code first, then migrate)
+- Migrations that assume a deploy boundary (old code + new schema = crash)
+- Missing feature flag to handle mixed old/new code during rolling deploy