chore: mark database codegen files as linguist-generated (#25787)

These files all have 'Code generated' / 'DO NOT EDIT' headers but
were not in .gitattributes. This causes GitHub to show them expanded
in PR diffs and count them toward diff stats.

Files added:
- coderd/database/queries.sql.go (sqlc)
- coderd/database/models.go, querier.go (sqlc)
- coderd/database/dbmock/dbmock.go (gomock)
- coderd/database/dbmetrics/querymetrics.go (dbgen)
- coderd/database/unique_constraint.go (dbgen)
- coderd/database/foreign_key_constraint.go (dbgen)
- coderd/database/check_constraint.go (dbgen)

.gitattributes

@@ -8,6 +8,21 @@ docs/reference/api/*.md linguist-generated=true
 docs/reference/cli/*.md linguist-generated=true
 coderd/apidoc/swagger.json linguist-generated=true
 coderd/database/dump.sql linguist-generated=true
+
+# Database codegen (sqlc)
+coderd/database/queries.sql.go linguist-generated=true
+coderd/database/models.go linguist-generated=true
+coderd/database/querier.go linguist-generated=true
+
+# Database codegen (gomock)
+coderd/database/dbmock/dbmock.go linguist-generated=true
+
+# Database codegen (dbgen)
+coderd/database/dbmetrics/querymetrics.go linguist-generated=true
+coderd/database/unique_constraint.go linguist-generated=true
+coderd/database/foreign_key_constraint.go linguist-generated=true
+coderd/database/check_constraint.go linguist-generated=true
+
 peerbroker/proto/*.go linguist-generated=true
 provisionerd/proto/*.go linguist-generated=true
 provisionerd/proto/version.go linguist-generated=false