Reapply "Build post chat and attachment workflows"

This reverts commit 5bc4a551af.

backend/migrations/005_refresh_tokens.sql

@@ -0,0 +1,11 @@
+CREATE TABLE IF NOT EXISTS refresh_tokens (
+  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  user_id UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+  token_hash TEXT NOT NULL UNIQUE,
+  expires_at TIMESTAMPTZ NOT NULL,
+  revoked_at TIMESTAMPTZ,
+  created_at TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+CREATE INDEX IF NOT EXISTS idx_refresh_tokens_user_id ON refresh_tokens (user_id);
+CREATE INDEX IF NOT EXISTS idx_refresh_tokens_token_hash ON refresh_tokens (token_hash);