Implement Phase 2 resilience improvements

Applies Phase 2 resilience improvements including slug uniqueness constraints, foreign key validation, and rate limiting. This includes new database migrations for slug uniqueness and foreign key validation, and updates to the edge function for rate limiting.

supabase/migrations/20251106235753_ac63e78e-ccd6-4f4f-b24b-fe168be7b001.sql

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+-- ============================================================================
+-- PHASE 2: RESILIENCE IMPROVEMENTS - Slug Uniqueness Constraints
+-- ============================================================================
+-- Add UNIQUE constraints on slug columns for companies and ride_models
+-- to prevent duplicate slugs and ensure data integrity
+
+-- Add unique constraint to companies.slug
+ALTER TABLE companies 
+  ADD CONSTRAINT companies_slug_unique UNIQUE (slug);
+
+-- Add unique constraint to ride_models.slug
+ALTER TABLE ride_models 
+  ADD CONSTRAINT ride_models_slug_unique UNIQUE (slug);
+
+-- Add indexes for performance (if they don't already exist)
+CREATE INDEX IF NOT EXISTS idx_companies_slug ON companies(slug);
+CREATE INDEX IF NOT EXISTS idx_ride_models_slug ON ride_models(slug);
+
+COMMENT ON CONSTRAINT companies_slug_unique ON companies IS 
+  'Ensures each company has a unique slug for URL routing';
+  
+COMMENT ON CONSTRAINT ride_models_slug_unique ON ride_models IS 
+  'Ensures each ride model has a unique slug for URL routing';