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/models.go

@@ -4225,6 +4225,7 @@ type ChatMessage struct {
 	ContentVersion      int16                 `db:"content_version" json:"content_version"`
 	TotalCostMicros     sql.NullInt64         `db:"total_cost_micros" json:"total_cost_micros"`
 	RuntimeMs           sql.NullInt64         `db:"runtime_ms" json:"runtime_ms"`
+	Deleted             bool                  `db:"deleted" json:"deleted"`
 }
 
 type ChatModelConfig struct {