feat: add Terms & Conditions checkbox to signup (refs #4)

- Add terms_agreements table for legal audit trail
- Create terms-agreement feature capsule with repository
- Modify signup to create terms agreement atomically
- Add checkbox with PDF link to SignupForm
- Capture IP, User-Agent, terms version, content hash
- Update CLAUDE.md documentation index

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/src/features/terms-agreement/migrations/001_create_terms_agreements.sql

@@ -0,0 +1,33 @@
+-- Terms Agreements Table
+-- Stores legal audit trail for Terms & Conditions acceptance at signup
+
+CREATE TABLE IF NOT EXISTS terms_agreements (
+  id UUID PRIMARY KEY DEFAULT uuid_generate_v4(),
+  user_id VARCHAR(255) NOT NULL,
+  agreed_at TIMESTAMP WITH TIME ZONE NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  ip_address VARCHAR(45) NOT NULL,
+  user_agent TEXT NOT NULL,
+  terms_version VARCHAR(50) NOT NULL,
+  terms_url VARCHAR(255) NOT NULL,
+  terms_content_hash VARCHAR(64) NOT NULL,
+  created_at TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP,
+  updated_at TIMESTAMP WITH TIME ZONE DEFAULT CURRENT_TIMESTAMP
+);
+
+-- Index for user lookup
+CREATE INDEX IF NOT EXISTS idx_terms_agreements_user_id ON terms_agreements(user_id);
+
+-- Trigger for updated_at
+CREATE OR REPLACE FUNCTION update_terms_agreements_updated_at()
+RETURNS TRIGGER AS $$
+BEGIN
+  NEW.updated_at = CURRENT_TIMESTAMP;
+  RETURN NEW;
+END;
+$$ LANGUAGE plpgsql;
+
+DROP TRIGGER IF EXISTS terms_agreements_updated_at ON terms_agreements;
+CREATE TRIGGER terms_agreements_updated_at
+  BEFORE UPDATE ON terms_agreements
+  FOR EACH ROW
+  EXECUTE FUNCTION update_terms_agreements_updated_at();