feat(filters): saved filters with a query language, and label management

"today & p1", "#Firma & @regnskab", "overdue | (today & assigned to: me)".
The grammar is Todoist's, so the import path from it does not require
relearning anything, and both languages' keywords are accepted.

It compiles to parameterised SQL rather than filtering in Go: one
indexed query instead of ten thousand rows crossing the boundary. No
typed text is ever concatenated into the statement — this is a language
people write by hand, which makes it exactly where string-building would
become an injection.

An expression is compiled before it is saved. A filter that cannot run
is worse than none: it is discovered later, as a list that is empty for
a reason nobody can see.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

internal/store/tasks.go

@@ -62,6 +62,12 @@ type TaskFilter struct {
 	CompletedOnly bool
 
 	Search string
+	// FilterSQL is a compiled saved-filter expression: a WHERE fragment over `t`
+	// with its own bound arguments. It is produced by the filterquery package and
+	// never assembled from user text here.
+	FilterSQL  string
+	FilterArgs []any
+
 	Limit  int
 	Offset int
 }
@@ -133,6 +139,10 @@ func (db *DB) ListTasks(ctx context.Context, userID string, f TaskFilter) ([]Tas
 		where = append(where, `t.rowid IN (SELECT rowid FROM tasks_fts WHERE tasks_fts MATCH ?)`)
 		args = append(args, expr)
 	}
+	if f.FilterSQL != "" {
+		where = append(where, "("+f.FilterSQL+")")
+		args = append(args, f.FilterArgs...)
+	}
 
 	query := `
 		SELECT t.id, t.project_id, t.section_id, t.parent_id, t.content, t.description,