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/querier.go

@@ -98,7 +98,6 @@ type sqlcQuerier interface {
 	// be recreated.
 	DeleteAllWebpushSubscriptions(ctx context.Context) error
 	DeleteApplicationConnectAPIKeysByUserID(ctx context.Context, userID uuid.UUID) error
-	DeleteChatMessagesAfterID(ctx context.Context, arg DeleteChatMessagesAfterIDParams) error
 	DeleteChatModelConfigByID(ctx context.Context, id uuid.UUID) error
 	DeleteChatProviderByID(ctx context.Context, id uuid.UUID) error
 	DeleteChatQueuedMessage(ctx context.Context, arg DeleteChatQueuedMessageParams) error
@@ -756,6 +755,8 @@ type sqlcQuerier interface {
 	// for the table.
 	// The CTE and the reorder is required because UPDATE doesn't guarantee order.
 	SelectUsageEventsForPublishing(ctx context.Context, now time.Time) ([]UsageEvent, error)
+	SoftDeleteChatMessageByID(ctx context.Context, id int64) error
+	SoftDeleteChatMessagesAfterID(ctx context.Context, arg SoftDeleteChatMessagesAfterIDParams) error
 	// Non blocking lock. Returns true if the lock was acquired, false otherwise.
 	//
 	// This must be called from within a transaction. The lock will be automatically