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.

internal/database/queries/queries.sql

@@ -2151,7 +2151,7 @@ SELECT * FROM collections WHERE id = $1;
 
 -- Get collections by user
 -- name: GetCollectionsByUser :many
-SELECT * FROM collections WHERE user_id = $1 ORDER BY created_at DESC;
+SELECT * FROM collections WHERE user_id = $1 AND deleted_at IS NULL ORDER BY created_at DESC;
 
 -- Get collection with book count
 -- name: GetCollectionWithBookCount :one
@@ -2208,7 +2208,8 @@ ORDER BY ci.added_at DESC;
 SELECT c.*
 FROM collections c
 JOIN collection_items ci ON c.id = ci.collection_id
-WHERE ci.media_item_id = $1;
+WHERE ci.media_item_id = $1
+  AND c.deleted_at IS NULL;
 
 -- Check if book is in collection
 -- name: IsBookInCollection :one
@@ -2485,6 +2486,7 @@ SELECT * FROM collections
 WHERE user_id = $1
   AND is_system_collection = true
   AND show_on_dashboard = true
+  AND deleted_at IS NULL
 ORDER BY priority ASC;
 
 -- name: GetUserCollectionsForDashboard :many
@@ -2492,6 +2494,7 @@ SELECT c.* FROM collections c
 WHERE c.user_id = $1
   AND c.show_on_dashboard = true
   AND c.is_system_collection = false
+  AND c.deleted_at IS NULL
 ORDER BY priority ASC;
 
 -- name: DeleteUserSystemCollection :exec
@@ -2500,6 +2503,38 @@ WHERE user_id = $1
   AND name = $2
   AND is_system_collection = true;
 
+-- System collections are permanent: identity is (user_id, query_type).
+-- Restore/reset operate IN PLACE (the row id — and everything referencing
+-- it: shelf mappings, prefs order, links — never churns). Deleted sections
+-- are TOMBSTONED (deleted_at) and stay invisible until restored.
+
+-- name: GetSystemCollectionByQueryType :one
+-- Unfiltered by design: restore/seed must resolve tombstoned rows too.
+SELECT * FROM collections
+WHERE user_id = $1
+  AND query_type = $2
+  AND is_system_collection = true;
+
+-- name: TombstoneSystemCollection :exec
+UPDATE collections SET deleted_at = NOW() WHERE id = $1;
+
+-- name: RestoreSystemCollectionRow :one
+-- Full reset, in place: canonical metadata, back on the dashboard,
+-- un-tombstoned. Item assignments are cleared by the caller ("full").
+UPDATE collections
+SET name = $2,
+    description = $3,
+    icon = $4,
+    color = $5,
+    priority = $6,
+    show_on_dashboard = true,
+    deleted_at = NULL
+WHERE id = $1
+RETURNING *;
+
+-- name: ClearCollectionItems :exec
+DELETE FROM collection_items WHERE collection_id = $1;
+
 -- name: CreateSystemCollection :one
 INSERT INTO collections (user_id, name, description, icon, color, show_on_dashboard, query_type, priority, is_system_collection, auto_assign_rules)
 VALUES ($1, $2, $3, $4, $5, $6, $7, $8, true, 'null'::jsonb)