feat(ai-chat): agent roles (admin-defined persona + optional model)

Reusable, workspace-shared agent roles for the built-in AI chat. A role is
a named persona (system-prompt instructions) + optional model override; a
chat is bound to a role at creation and applies it every turn.

Backend:
- migration 20260620T120000: ai_agent_roles table + ai_chats.role_id
  (FK ON DELETE SET NULL); hand-merged types into db.d.ts/entity.types.ts
  (db.d.ts is hand-curated here, full codegen would clobber it).
- core/ai-chat/roles: CRUD module. list = any workspace member; create/
  update/delete = admin (Manage Settings ability, like ai-settings/mcp).
  All repo queries scoped by workspace_id; soft-delete (deleted_at).
- buildSystemPrompt gains roleInstructions: role REPLACES the persona base
  (admin prompt / DEFAULT_PROMPT) but SAFETY_FRAMEWORK + context are always
  still appended.
- stream(): role resolved from ai_chats.role_id for existing chats (never
  the request body -> no per-turn role swap); body.roleId only on creation.
  Disabled (enabled=false) and soft-deleted roles fall back to universal.
- getChatModel(workspaceId, override): role model_config can swap model id /
  driver; a driver without configured creds throws 503 with a clear message
  naming the driver+role, resolved BEFORE response hijack.

Client:
- new-chat role picker (enabled roles only, default Universal assistant),
  roleId sent only on the first message; role badge (emoji+name) in the chat
  header and conversation list; admin Agent-roles management section in
  Settings -> AI (add/edit/delete, MCP-form pattern).

Tests: ai-chat.prompt.spec (role layering + safety always present, incl.
jailbreak); ai.service.spec (override on unconfigured driver -> 503).

Implements docs/ai-agent-roles-plan.md.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/server/src/core/ai-chat/ai-chat.controller.ts

@@ -142,10 +142,16 @@ export class AiChatController {
 
     const body = (req.body ?? {}) as AiChatStreamBody;
 
-    // Resolve the model BEFORE hijack so an unconfigured provider returns a
-    // clean JSON 503 (AiNotConfiguredException is a 503 HttpException; letting
-    // it propagate here yields a normal response, not a broken stream).
-    const model = await this.aiChatService.getChatModel(workspace.id);
+    // Resolve the agent role for this turn BEFORE hijack: existing chats read it
+    // from ai_chats.role_id (authoritative), a new chat from body.roleId. The
+    // role drives both the persona and the optional model override below.
+    const role = await this.aiChatService.resolveRoleForRequest(workspace, body);
+
+    // Resolve the model (applying the role's optional override) BEFORE hijack so
+    // an unconfigured provider — including a role pointing at an unconfigured
+    // driver — returns a clean JSON 503 (AiNotConfiguredException is a 503
+    // HttpException) instead of breaking mid-stream.
+    const model = await this.aiChatService.getChatModel(workspace.id, role);
 
     // Abort the agent loop when the client disconnects. `close` also fires on
     // normal completion, so only abort when the response has not finished
@@ -173,6 +179,7 @@ export class AiChatController {
         res,
         signal: controller.signal,
         model,
+        role,
       });
     } catch (err) {
       // Any failure AFTER hijack can no longer send a clean JSON error, so emit