fix(db): add migration to create shared_links table for databases that skipped 025

Migration 025_add_shared_links was inserted into the Alembic chain
(between 024_add_api_tokens and 025_add_user_notifications) after some
databases had already been migrated past that point. Those databases
never had the shared_links table created, causing OperationalError when
the expire-shared-links scheduled task runs or when users try to create
shared links.

This commit:
- Adds migration 027_ensure_shared_links_table that idempotently creates
  the table if it doesn't exist
- Updates migrations/env.py to import all models for autogenerate support
- Adds shared_links to db_migrate.py _TABLE_ORDER for proper migration
  ordering
- Adds a regression test verifying the fix

Co-authored-by: christianlouis <361235+christianlouis@users.noreply.github.com>

migrations/env.py

@@ -20,13 +20,28 @@ from app.database import Base
 
 # Ensure all models are imported so Base.metadata is populated.
 from app.models import (  # noqa: F401
+    ApiToken,
     ApplicationSettings,
+    BackupRecord,
     DocumentMetadata,
     FileProcessingStep,
     FileRecord,
+    InAppNotification,
+    LocalUser,
+    Pipeline,
+    PipelineStep,
     ProcessingLog,
     SavedSearch,
+    ScheduledJob,
     SettingsAuditLog,
+    SharedLink,
+    SubscriptionPlan,
+    UserImapAccount,
+    UserIntegration,
+    UserNotificationPreference,
+    UserNotificationTarget,
+    UserProfile,
+    WebhookConfig,
 )
 
 # Alembic Config object – provides access to values in alembic.ini.