feat(ai-chat): durable detached agent runs — phase 1 (#184/#234)

Squashed for a clean rebase onto develop (was 19 commits; the reviewer approved
the net diff at fb246080). Detaches an agent run from the HTTP request/browser
window: a run is a first-class lifecycle object (ai_chat_runs), a browser
disconnect no longer kills it, a concurrent-run insert-gate prevents double runs,
and a reopened chat live-follows a still-running run via a polled observer merge.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/server/src/database/database.module.ts

@@ -31,6 +31,7 @@ import { FavoriteRepo } from '@docmost/db/repos/favorite/favorite.repo';
 import { TemplateRepo } from '@docmost/db/repos/template/template.repo';
 import { AiChatRepo } from '@docmost/db/repos/ai-chat/ai-chat.repo';
 import { AiChatMessageRepo } from '@docmost/db/repos/ai-chat/ai-chat-message.repo';
+import { AiChatRunRepo } from '@docmost/db/repos/ai-chat/ai-chat-run.repo';
 import { AiChatPageSnapshotRepo } from '@docmost/db/repos/ai-chat/ai-chat-page-snapshot.repo';
 import { AiProviderCredentialsRepo } from '@docmost/db/repos/ai-chat/ai-provider-credentials.repo';
 import { AiMcpServerRepo } from '@docmost/db/repos/ai-chat/ai-mcp-server.repo';
@@ -105,6 +106,7 @@ import { normalizePostgresUrl } from '../common/helpers';
     TemplateRepo,
     AiChatRepo,
     AiChatMessageRepo,
+    AiChatRunRepo,
     AiChatPageSnapshotRepo,
     AiProviderCredentialsRepo,
     AiMcpServerRepo,
@@ -139,6 +141,7 @@ import { normalizePostgresUrl } from '../common/helpers';
     TemplateRepo,
     AiChatRepo,
     AiChatMessageRepo,
+    AiChatRunRepo,
     AiChatPageSnapshotRepo,
     AiProviderCredentialsRepo,
     AiMcpServerRepo,