step 4: fresh kernel-only migrations

Deleted all 23+23 old chat-switchboard migration files.
Wrote 9+9 clean kernel-only migrations in postgres/ and sqlite/ subdirs.

27 tables per dialect covering all 20 store interfaces:
  001: users, refresh_tokens, policies, settings, presence, oidc
  002: teams, team_members, groups, group_members
  003: packages, package_user_settings, ext_permissions, ext_data_tables
  004: ext_connections, ext_dependencies, resource_grants
  005: notifications, notification_preferences
  006: audit_log
  007: workflows, workflow_stages, workflow_versions
  008: tasks, task_runs
  009: ws_tickets, rate_limit_counters

Dropped: providers, personas, channels, messages, knowledge, notes,
memory, workspaces, projects, folders, files, usage_log, tool_health,
workflow_assignments, ext_view_channels.

Schema changes vs old:
  - pgvector extension removed (no KB embeddings)
  - resource_grants: resource_type CHECK removed (open-ended)
  - workflow_stages: persona_id kept as nullable TEXT (no FK)
  - workflow_stages: stage_mode default=form_only, added custom
  - tasks: dropped output_channel_id, provider_config_id columns
  - tasks: task_type removed prompt, output_mode: notification|webhook|log
  - task_runs: dropped channel_id
  - platform_policies: kernel-only seeds
  - global_settings: kernel-only seeds, site name=Switchboard Core
  - Everyone group: kernel permissions (extension.use, workflow.submit)

-2815/+487 lines.

server/database/migrations/postgres/005_notifications.sql

@@ -0,0 +1,31 @@
+-- ==========================================
+-- Switchboard Core — 005 Notifications
+-- ==========================================
+
+CREATE TABLE IF NOT EXISTS notifications (
+    id              UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    user_id         UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    type            VARCHAR(50) NOT NULL,
+    title           VARCHAR(255) NOT NULL,
+    body            TEXT DEFAULT '',
+    resource_type   VARCHAR(50),
+    resource_id     UUID,
+    is_read         BOOLEAN DEFAULT false,
+    created_at      TIMESTAMPTZ DEFAULT NOW()
+);
+
+CREATE INDEX IF NOT EXISTS idx_notifications_user_unread
+    ON notifications(user_id, is_read, created_at DESC);
+CREATE INDEX IF NOT EXISTS idx_notifications_user_created
+    ON notifications(user_id, created_at DESC);
+
+CREATE TABLE IF NOT EXISTS notification_preferences (
+    id          UUID PRIMARY KEY DEFAULT gen_random_uuid(),
+    user_id     UUID NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    type        VARCHAR(50) NOT NULL,
+    in_app      BOOLEAN NOT NULL DEFAULT true,
+    email       BOOLEAN NOT NULL DEFAULT false,
+    UNIQUE(user_id, type)
+);
+
+CREATE INDEX IF NOT EXISTS idx_notification_prefs_user ON notification_preferences(user_id);