Reapply "Build post chat and attachment workflows"

This reverts commit 5bc4a551af.

backend/migrations/005_refresh_tokens.sql

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+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);

backend/migrations/006_calendar_item_chat_messages.sql

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+CREATE TABLE IF NOT EXISTS calendar_item_chat_messages (
+  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  calendar_item_id UUID NOT NULL REFERENCES calendar_items(id) ON DELETE CASCADE,
+  channel TEXT NOT NULL CHECK (channel IN ('external', 'internal')),
+  body TEXT NOT NULL,
+  created_by UUID REFERENCES users(id) ON DELETE SET NULL,
+  created_at TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+CREATE INDEX IF NOT EXISTS idx_calendar_item_chat_messages_item_channel
+  ON calendar_item_chat_messages (calendar_item_id, channel, created_at);

backend/migrations/007_chat_message_features.sql

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+ALTER TABLE calendar_item_chat_messages
+ADD COLUMN IF NOT EXISTS edited_at TIMESTAMPTZ,
+ADD COLUMN IF NOT EXISTS deleted_at TIMESTAMPTZ;
+
+CREATE TABLE IF NOT EXISTS calendar_item_chat_attachments (
+  id UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+  chat_message_id UUID NOT NULL REFERENCES calendar_item_chat_messages(id) ON DELETE CASCADE,
+  original_filename TEXT NOT NULL,
+  stored_filename TEXT NOT NULL,
+  mime_type TEXT NOT NULL,
+  size_bytes BIGINT NOT NULL CHECK (size_bytes >= 0),
+  storage_driver TEXT NOT NULL,
+  storage_path TEXT NOT NULL,
+  created_by UUID REFERENCES users(id) ON DELETE SET NULL,
+  created_at TIMESTAMPTZ NOT NULL DEFAULT now()
+);
+
+CREATE INDEX IF NOT EXISTS idx_calendar_item_chat_attachments_message
+  ON calendar_item_chat_attachments (chat_message_id, created_at);