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/queries/chats.sql

@@ -5,13 +5,23 @@ WHERE id = @id OR root_chat_id = @id;
 -- name: UnarchiveChatByID :exec
 UPDATE chats SET archived = false, updated_at = NOW() WHERE id = @id::uuid;
 
--- name: DeleteChatMessagesAfterID :exec
-DELETE FROM
+-- name: SoftDeleteChatMessagesAfterID :exec
+UPDATE
     chat_messages
+SET
+    deleted = true
 WHERE
     chat_id = @chat_id::uuid
     AND id > @after_id::bigint;
 
+-- name: SoftDeleteChatMessageByID :exec
+UPDATE
+    chat_messages
+SET
+    deleted = true
+WHERE
+    id = @id::bigint;
+
 -- name: GetChatByID :one
 SELECT
     *
@@ -26,7 +36,8 @@ SELECT
 FROM
     chat_messages
 WHERE
-    id = @id::bigint;
+    id = @id::bigint
+    AND deleted = false;
 
 -- name: GetChatMessagesByChatID :many
 SELECT
@@ -37,6 +48,7 @@ WHERE
     chat_id = @chat_id::uuid
     AND id > @after_id::bigint
     AND visibility IN ('user', 'both')
+    AND deleted = false
 ORDER BY
     created_at ASC;
 
@@ -52,6 +64,7 @@ WHERE
         ELSE true
     END
     AND visibility IN ('user', 'both')
+    AND deleted = false
 ORDER BY
     id DESC
 LIMIT
@@ -66,6 +79,7 @@ WITH latest_compressed_summary AS (
     WHERE
         chat_id = @chat_id::uuid
         AND compressed = TRUE
+        AND deleted = false
         AND visibility = 'model'
     ORDER BY
         created_at DESC,
@@ -80,6 +94,7 @@ FROM
 WHERE
     chat_id = @chat_id::uuid
     AND visibility IN ('model', 'both')
+    AND deleted = false
     AND (
         (
             role = 'system'
@@ -496,6 +511,7 @@ FROM
 WHERE
     chat_id = @chat_id::uuid
     AND role = @role::chat_message_role
+    AND deleted = false
 ORDER BY
     created_at DESC, id DESC
 LIMIT