feat(ai-chat): notify the agent of user page edits between turns (closes #274)

The agent rebuilds context from DB each turn and didn't know the user manually
edited the open page since its last response, so it could overwrite those edits.
Add a per-turn ephemeral <page_changed> note in the system prompt (twin of
INTERRUPT_NOTE, self-clearing) carrying a unified Markdown diff of what changed
since the END of the agent's previous turn.

- New ai_chat_page_snapshots table (migration + hand-declared db.d.ts/entity
  types) storing the page Markdown per (chat,page) at each turn's end.
- Pure computePageChange util (whitespace-normalized unified diff via the
  existing jsdiff dep, 6KB cap + getPage hint).
- Turn start: if the open page's updatedAt moved past the snapshot, diff current
  vs snapshot; non-empty -> PAGE_CHANGED_NOTE in the safety sandwich.
- Turn end: upsert the snapshot on EVERY terminal path (onFinish/onError/onAbort,
  once) so the agent's own edits are excluded by construction even on aborted
  turns.
All best-effort (never breaks/latency-regresses a turn); fast path when updatedAt
is unchanged. Server-only.

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 { 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';
 import { AiAgentRoleRepo } from '@docmost/db/repos/ai-agent-roles/ai-agent-roles.repo';
@@ -104,6 +105,7 @@ import { normalizePostgresUrl } from '../common/helpers';
     TemplateRepo,
     AiChatRepo,
     AiChatMessageRepo,
+    AiChatPageSnapshotRepo,
     AiProviderCredentialsRepo,
     AiMcpServerRepo,
     AiAgentRoleRepo,
@@ -137,6 +139,7 @@ import { normalizePostgresUrl } from '../common/helpers';
     TemplateRepo,
     AiChatRepo,
     AiChatMessageRepo,
+    AiChatPageSnapshotRepo,
     AiProviderCredentialsRepo,
     AiMcpServerRepo,
     AiAgentRoleRepo,