Add user_credentials migration

.go-version

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+1.23.6

express/framework/migrations/2026-01-01_03-user-credentials.sql

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+-- 0003_user_credentials.sql
+-- Create user_credentials table for password storage (extensible for other auth methods)
+
+CREATE TABLE user_credentials (
+    id UUID PRIMARY KEY,
+    user_id UUID NOT NULL REFERENCES users(id),
+    credential_type TEXT NOT NULL DEFAULT 'password',
+    password_hash TEXT,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+-- Each user can have at most one credential per type
+CREATE UNIQUE INDEX user_credentials_user_type_idx ON user_credentials (user_id, credential_type);
+
+-- Index for user lookups
+CREATE INDEX user_credentials_user_id_idx ON user_credentials (user_id);