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/sqlite/005_notifications.sql

@@ -0,0 +1,31 @@
+-- ==========================================
+-- Switchboard Core — 005 Notifications (SQLite)
+-- ==========================================
+
+CREATE TABLE IF NOT EXISTS notifications (
+    id              TEXT PRIMARY KEY,
+    user_id         TEXT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    type            TEXT NOT NULL,
+    title           TEXT NOT NULL,
+    body            TEXT DEFAULT '',
+    resource_type   TEXT,
+    resource_id     TEXT,
+    is_read         INTEGER DEFAULT 0,
+    created_at      TEXT DEFAULT (datetime('now'))
+);
+
+CREATE INDEX IF NOT EXISTS idx_notifications_user_unread
+    ON notifications(user_id, is_read, created_at);
+CREATE INDEX IF NOT EXISTS idx_notifications_user_created
+    ON notifications(user_id, created_at);
+
+CREATE TABLE IF NOT EXISTS notification_preferences (
+    id          TEXT PRIMARY KEY,
+    user_id     TEXT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    type        TEXT NOT NULL,
+    in_app      INTEGER NOT NULL DEFAULT 1,
+    email       INTEGER NOT NULL DEFAULT 0,
+    UNIQUE(user_id, type)
+);
+
+CREATE INDEX IF NOT EXISTS idx_notification_prefs_user ON notification_preferences(user_id);