…seed PG connection saturation fix

Closes task #88 per PM @不穷 dispatch (msg=8f130f25). Implements task
#61 spec v1 § 2.4 P2-S1 + Planetegg P2-HIGH (msg=db7fb085 +
msg=1314ac59) + Singapore SRE diagnostic (Planetegg msg=4043adf4)
batch alias resolution.

Background
----------
``LineageGraphStoreWithAliasRedirect.expand_neighbors_n_hops`` is on
the ``GET /api/v2/collections/{id}/graphs`` and ``/graphs/hybrid``
read paths. Pre-fix, it called ``AliasMapRepository.resolve_canonical``
once per anchor name via ``asyncio.gather``, which checks out one
PG connection per name. Spec § 2.4 P2-S1 quantification:

* ``GET /graphs?max_nodes=1000`` → up to **2 × max_nodes = 2000**
  seeds.
* ``GET /graphs/hybrid``: default 1000 / max 5000 seeds.

At those cardinalities the PG connection pool saturates, observed
in Singapore production (Planetegg msg=4043adf4 SRE diagnostic).

Changes
-------
* ``aperag/graph_curation/alias_map.py``: new
  :meth:`AliasMapRepository.resolve_canonical_many` batch primitive.
  Single SQL ``SELECT alias_name, canonical_name FROM
  aperag_lineage_entity_alias WHERE collection_id=? AND alias_name
  IN (...)`` reads all matching rows in one shot. Names absent from
  the result set fall back to themselves (mirrors single-name
  ``resolve_canonical`` semantics). Empty / falsy names short-
  circuit without an SQL lookup. Total connections checked out: **1**
  per call regardless of seed count. Caller order is preserved on
  the dict iteration order (insertion order semantics).
* ``aperag/indexing/alias_redirect_store.py``: rewrite
  ``LineageGraphStoreWithAliasRedirect.expand_neighbors_n_hops`` to
  use the batch primitive. ``asyncio.gather`` per-name fan-out gone;
  ``import asyncio`` no longer needed at module top-level.
* Test stub ``_FakeAliasRepo`` in
  ``tests/unit_test/indexing/test_alias_redirect_store.py``: now
  implements both ``resolve_canonical`` (single, used by
  upsert/get/delete redirect paths) and ``resolve_canonical_many``
  (batch, used by expand) + tracks call counts so tests can pin the
  call-graph (i.e. expand path goes through batch primitive exactly
  once).

Tests
-----
* ``tests/unit_test/graph_curation/test_alias_map.py`` (7 new):
  - ``test_resolve_canonical_many_returns_self_for_unmapped_names``
  - ``test_resolve_canonical_many_mixed_alias_and_canonical``
  - ``test_resolve_canonical_many_dedupes_input``
  - ``test_resolve_canonical_many_empty_input``
  - ``test_resolve_canonical_many_handles_empty_string``
  - ``test_resolve_canonical_many_per_collection_isolation``
  - ``test_resolve_canonical_many_large_seed_cap`` (2000-name spec
    quantification — pinned correctness at the spec-cap so a future
    regression that re-introduces per-name fan-out either times out
    or breaks the result shape).
* ``tests/unit_test/indexing/test_alias_redirect_store.py`` (2 new):
  - ``test_expand_neighbors_uses_batch_alias_resolution`` —
    call-graph gate: exactly 1 ``resolve_canonical_many`` call,
    zero ``resolve_canonical`` calls, regardless of seed count. A
    regression that re-introduces the gather pattern is caught
    immediately.
  - ``test_expand_neighbors_large_seed_cap_uses_single_batch_call``
    — 2000-seed spec-cap pinned at the call-graph level.

Local: ``uv run pytest tests/unit_test/graph_curation/
tests/unit_test/indexing/test_alias_redirect_store.py`` →
**56 passed, 1 warning**.

Spec / scope alignment
----------------------
* task #61 spec v1 § 2.4 P2-S1 — batch resolve primitive ✅
* task #61 spec v1 § 2.4 P2-S2 — ``expand_neighbors_n_hops`` seed
  cap test ✅
* Lesson #17 backend 收敛 contract — single primitive replaces N-
  parallel fan-out at the same caller, no FE / API changes
  required ✅
* Lesson #18 mechanical gate codification — call-graph assertion in
  the redirect-store test is the mechanical gate (caught by CI on
  any future regression that bypasses the batch primitive) ✅

Follow-ups (NOT in this PR)
---------------------------
* P3 cross-cutting concern: every ``LineageGraphStore`` consumer
  that currently invokes the alias path per-name (e.g. some
  GraphCurationService internals) should migrate to the batch
  primitive — independent task gated on production data showing
  the residual N-fan-out is a real bottleneck.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>