chore: rename user-facing AI Bridge strings to AI Gateway (#26700)

Rename user-facing "AI Bridge" strings to "AI Gateway" in deployment
config, RBAC display names, log messages, error strings, docs style
guide, and Grafana dashboard README.

Deprecated option names and descriptions (the `--aibridge-*` block) are
intentionally kept as "AI Bridge". The `Name` field cannot be renamed
because `serpent` uses it as a unique key during JSON serialization;
duplicating names causes `UnmarshalJSON` failures (e.g. in the support
bundle). Descriptions also stay as "AI Bridge" to avoid confusion
between the deprecated and primary options.

Refs https://linear.app/codercom/issue/AIGOV-226

> Generated with the assistance of Coder Agents (@ssncferreira)

coderd/database/dbauthz/dbauthz.go

@@ -638,12 +638,12 @@ var (
 	// See aibridged package.
 	subjectAibridged = rbac.Subject{
 		Type:         rbac.SubjectAibridged,
-		FriendlyName: "AI Bridge Daemon",
+		FriendlyName: "AI Gateway Daemon",
 		ID:           uuid.Nil.String(),
 		Roles: rbac.Roles([]rbac.Role{
 			{
 				Identifier:  rbac.RoleIdentifier{Name: "aibridged"},
-				DisplayName: "AI Bridge Daemon",
+				DisplayName: "AI Gateway Daemon",
 				Site: rbac.Permissions(map[string][]policy.Action{
 					rbac.ResourceUser.Type: {
 						policy.ActionRead,         // Required to validate API key owner is active.