Revert "chore(db): remove deprecated entity schema from memory_links" (#3177)

[nicoloboschi]

Reverts #3177 (squash commit 5f8a030).

Why

Migration c1e7a9d3f5b2 collapses idx_memory_links_unique — which means rebuilding the unique index on memory_links. On large established banks that table (and index) is huge, and even with CONCURRENTLY/ONLINE the rebuild is a long, expensive operation we don't want to impose for what was a pure schema cleanup with no functional benefit.

#3177 was never shipped in a core release (latest is v0.8.6, cut 2026-07-29), so reverting now means released environments never run it.

What this PR does

• git revert of the squash commit: restores the memory_links entity schema plumbing (MemoryLink.entity_id, write-path code, tests) and deletes migration c1e7a9d3f5b2 and its migration test.
• Fixes the Alembic chain: f2a6d8c4b1e9 (drop stale global vector index) chained off the deleted revision; it is re-parented onto e4a7c1b9d2f6 (drop access_count). Chain now resolves to the single head b3e8d1c6f4a9 (verified by walking all 94 revisions).

Upgrade path from v0.8.6 is now … → e4a7c1b9d2f6 → f2a6d8c4b1e9 → b3e8d1c6f4a9 — the entity-schema drop is simply gone from the tree, so upgraders never execute it.

Caveat for environments tracking main

Any environment that started the API between #3177's merge (2026-08-04) and this revert has already applied c1e7a9d3f5b2 (migrations run on startup):

• If its alembic_version has since advanced past it, migrations still resolve, but the schema is missing memory_links.entity_id while the reverted code writes that column — it needs a manual re-add of the entity schema (or a DB reset for dev envs).
• If its alembic_version is exactly c1e7a9d3f5b2, Alembic will fail with "Can't locate revision" until the version row is stamped to e4a7c1b9d2f6.

This only affects main-trackers from the last 3 days; no released environment is impacted.

Testing

• tests/test_migration_shape.py + tests/test_link_utils.py pass (215 passed).
• ./scripts/hooks/lint.sh clean.
• Alembic script directory resolves to a single head with no dangling revisions.