feat(server): system collections become permanent — soft-delete + in-place restore + query_type-keyed provisioning

The old model hard-deleted system collections and re-created them at
the tail, resurrecting stale dedups and breaking ordering. Redesign
(user-directed B-final):

- schema.sql: collections.deleted_at TIMESTAMPTZ (idempotent ALTER —
  the whole schema is re-applied on every boot via executeSchema).
- queries.sql: every collection list/read query filters
  deleted_at IS NULL; new GetSystemCollectionByQueryType :one
  (unfiltered, sees tombstones), TombstoneSystemCollection :exec,
  ClearCollectionItems :exec, RestoreSystemCollectionRow :one
  (in-place metadata reset + un-tombstone; collections have no
  updated_at column — ResetUpdatedAt was a 500 caught by tests).
  sqlc regenerated.
- collection_service.DeleteCollection: system collections are
  tombstoned (preserving position/ownership); user collections keep
  the hard delete. No ErrNoRows to the client (delete is idempotent).
- dashboard_service.RestoreSystemCollection: resolve by query_type;
  found → in-place restore (ClearCollectionItems + metadata reset,
  un-tombstoned); missing → provision fresh. "Want to Read"/etc.
  stay at their home positions forever.
- handlers/collections.CreateCollection: name-taken is 409, not 500.
- handlers/auth.CreateDefaultCollectionsForUser: idempotent by
  (user_id, query_type) — update-in-place when a row for that query
  type exists (heals renames/tombstones), insert when missing; this
  also fixes the duplicate-system-collections regression at source.

database/schema/schema.sql

@@ -1135,9 +1135,15 @@ CREATE TABLE IF NOT EXISTS collections (
     priority INT DEFAULT 100,
     is_system_collection BOOLEAN DEFAULT false,
     created_at TIMESTAMP WITH TIME ZONE DEFAULT NOW(),
+    deleted_at TIMESTAMP WITH TIME ZONE,
     UNIQUE(user_id, name)
 );
 
+-- System collections (the 5 dashboard sections) are permanent: "delete"
+-- tombstones the row (deleted_at) and restore un-tombstones it in place —
+-- the row id (and everything referencing it) never churn.
+ALTER TABLE collections ADD COLUMN IF NOT EXISTS deleted_at TIMESTAMPTZ;
+
 CREATE INDEX IF NOT EXISTS idx_collections_user_id ON collections(user_id);
 
 -- Index for dashboard queries