perf(migrations): skippable extension reconcile + drop unused global vector index

[nicoloboschi]

Summary

Two changes that trim the post-migration extension reconcile
(ensure_vector_extension / ensure_text_search_extension), which runs per
tenant schema and dominates the migration job at large tenant counts.

1. --skip-extension-reconcile on run-db-migration

The reconcile only does real work when the configured backend
(HINDSIGHT_API_VECTOR_EXTENSION / HINDSIGHT_API_TEXT_SEARCH_EXTENSION) differs
from a schema's existing index/column shape — a rare, operator-driven change (and
on populated tables it refuses and asks you to re-embed anyway). On a routine
no-change re-migration over many tenants it's pure per-tenant overhead.

run_migrations_for_schemas already takes an ensure_extensions parameter; this
exposes it on the CLI as --skip-extension-reconcile (default off → behavior
unchanged). A backend change still needs a normal run to reshape the indexes —
the flag is an explicit "I haven't changed the backend" assertion.

2. Stop creating the unused global memory_units vector index

For per-bank backends (pgvector / pgvectorscale / vchord), every vector search is
bank + fact_type scoped and served by the per-(bank, fact_type) partial indexes
created at bank-creation time (bank_utils.create_bank_vector_indexes). The
global idx_memory_units_embedding is never chosen by the planner when bank_id
is in the WHERE clause — which is exactly why migration d5e6f7a8b9c0 drops it
for these backends.

ensure_vector_extension nonetheless recreated that global index on a fresh/empty
schema (the "No embedding index found, will create it" path), putting an unused,
write-amplifying index back on every schema. This skips creating it for per-bank
backends. scann (which genuinely uses a global filtered index) is unaffected.

Verified via EXPLAIN on a real DB (3000 rows, both indexes present): the
semantic query uses the per-bank idx_mu_emb_* index, and dropping the global
index leaves the plan identical.

Tests

• Parametrized check that --skip-extension-reconcile threads ensure_extensions
  through to run_migrations_for_schemas.
• A pg0 integration test asserting a fresh pgvector schema run through
  migrate + ensure_vector_extension ends up with no global
  idx_memory_units_embedding (fails without the fix).