Move some of the responsibility to the caller to make it clearer what is
being used

…able

All callers used the name of the table that was passed in, except for the
use in `copy.rs`, but copying doesn't change table names, so the src and
dst names of tables are the same if they exist in the dst

Also, test some variations of the same index definition

Consolidate index creation into a single `Table::indexes()` method that
returns all indexes (time-travel, attribute, aggregate) as `CreateIndex`
objects. This replaces the old string-based methods and eliminates the
`index_def: Option<IndexList>` parameter threading through the codebase.

Key changes:
- Add `Table::indexes()` combining time_travel + attribute + aggregate indexes
- Add `attr_index_spec()` and `add_attribute_indexes()` structured helpers
- Move env var check into `CreateIndex::to_postpone()` so callers need not check
- Simplify `Table::as_ddl()` to iterate indexes with postpone filtering
- Remove old `create_time_travel_indexes`, `create_attribute_indexes`,
  `create_postponed_indexes`, `create_aggregate_indexes` string methods
- Remove `index_def` parameter from Layout, DeploymentStore, SubgraphStore
- Update copy.rs to use `indexes()` + `references_column_not_in()` for new fields
- Update prune.rs to use simplified `as_ddl()` without index_def
- Update all DDL test constants for new single-line index format

Add a trigger that creates postponed indexes when a subgraph gets
within a configurable number of blocks (default 10000) of the chain
head. This ensures indexes are in place before the subgraph starts
serving queries.

The new env var GRAPH_POSTPONE_INDEXES_CREATION_THRESHOLD controls
how many blocks before the chain head to trigger index creation. The
creation is idempotent (IF NOT EXISTS + CONCURRENTLY) and only
attempted once per subgraph run via an AtomicBool guard.

Replace the IndexList-based `recreate_invalid_indexes` call in
`start_subgraph()` with a call to `create_postponed_indexes()`. This
uses `IF NOT EXISTS` and `CONCURRENTLY` to safely create any missing
postponed indexes on every restart, acting as a safety net.

Remove the now-unused `IndexList::recreate_invalid_indexes` method.