feat: add deleted flag to chat messages for soft-delete (#23223)

Adds a `deleted` boolean column to the `chat_messages` table. Messages
are never physically deleted from the database — instead they are marked
as deleted so that usage and cost data is preserved.

## Changes

### Migration
- New migration (000444) adds `deleted boolean NOT NULL DEFAULT false`
to `chat_messages`

### SQL queries
- `DeleteChatMessagesAfterID` → `SoftDeleteChatMessagesAfterID` (UPDATE
SET deleted=true instead of DELETE)
- New `SoftDeleteChatMessageByID` query for single-message soft-delete
- All read queries now filter `deleted = false`:
  - `GetChatMessageByID`
  - `GetChatMessagesByChatID`
  - `GetChatMessagesByChatIDDescPaginated`
  - `GetChatMessagesForPromptByChatID` (both CTE and main query)
  - `GetLastChatMessageByRole`
- Cost/usage queries (`GetChatCostSummary`, `GetChatCostPerModel`, etc.)
intentionally still include deleted messages to preserve accurate spend
tracking

### EditMessage behavior
- Previously: updated the message content in-place + hard-deleted
subsequent messages
- Now: soft-deletes the original message + soft-deletes subsequent
messages + inserts a new message with the updated content
- This preserves the original message data (tokens, cost, content) in
the database

coderd/database/dbmock/dbmock.go

@@ -598,20 +598,6 @@ func (mr *MockStoreMockRecorder) DeleteApplicationConnectAPIKeysByUserID(ctx, us
 	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "DeleteApplicationConnectAPIKeysByUserID", reflect.TypeOf((*MockStore)(nil).DeleteApplicationConnectAPIKeysByUserID), ctx, userID)
 }
 
-// DeleteChatMessagesAfterID mocks base method.
-func (m *MockStore) DeleteChatMessagesAfterID(ctx context.Context, arg database.DeleteChatMessagesAfterIDParams) error {
-	m.ctrl.T.Helper()
-	ret := m.ctrl.Call(m, "DeleteChatMessagesAfterID", ctx, arg)
-	ret0, _ := ret[0].(error)
-	return ret0
-}
-
-// DeleteChatMessagesAfterID indicates an expected call of DeleteChatMessagesAfterID.
-func (mr *MockStoreMockRecorder) DeleteChatMessagesAfterID(ctx, arg any) *gomock.Call {
-	mr.mock.ctrl.T.Helper()
-	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "DeleteChatMessagesAfterID", reflect.TypeOf((*MockStore)(nil).DeleteChatMessagesAfterID), ctx, arg)
-}
-
 // DeleteChatModelConfigByID mocks base method.
 func (m *MockStore) DeleteChatModelConfigByID(ctx context.Context, id uuid.UUID) error {
 	m.ctrl.T.Helper()
@@ -7112,6 +7098,34 @@ func (mr *MockStoreMockRecorder) SelectUsageEventsForPublishing(ctx, now any) *g
 	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "SelectUsageEventsForPublishing", reflect.TypeOf((*MockStore)(nil).SelectUsageEventsForPublishing), ctx, now)
 }
 
+// SoftDeleteChatMessageByID mocks base method.
+func (m *MockStore) SoftDeleteChatMessageByID(ctx context.Context, id int64) error {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "SoftDeleteChatMessageByID", ctx, id)
+	ret0, _ := ret[0].(error)
+	return ret0
+}
+
+// SoftDeleteChatMessageByID indicates an expected call of SoftDeleteChatMessageByID.
+func (mr *MockStoreMockRecorder) SoftDeleteChatMessageByID(ctx, id any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "SoftDeleteChatMessageByID", reflect.TypeOf((*MockStore)(nil).SoftDeleteChatMessageByID), ctx, id)
+}
+
+// SoftDeleteChatMessagesAfterID mocks base method.
+func (m *MockStore) SoftDeleteChatMessagesAfterID(ctx context.Context, arg database.SoftDeleteChatMessagesAfterIDParams) error {
+	m.ctrl.T.Helper()
+	ret := m.ctrl.Call(m, "SoftDeleteChatMessagesAfterID", ctx, arg)
+	ret0, _ := ret[0].(error)
+	return ret0
+}
+
+// SoftDeleteChatMessagesAfterID indicates an expected call of SoftDeleteChatMessagesAfterID.
+func (mr *MockStoreMockRecorder) SoftDeleteChatMessagesAfterID(ctx, arg any) *gomock.Call {
+	mr.mock.ctrl.T.Helper()
+	return mr.mock.ctrl.RecordCallWithMethodType(mr.mock, "SoftDeleteChatMessagesAfterID", reflect.TypeOf((*MockStore)(nil).SoftDeleteChatMessagesAfterID), ctx, arg)
+}
+
 // TryAcquireLock mocks base method.
 func (m *MockStore) TryAcquireLock(ctx context.Context, pgTryAdvisoryXactLock int64) (bool, error) {
 	m.ctrl.T.Helper()