* perf: add GIN index on source_memory_ids for observation lookup
Addresses a 927x performance regression (45ms → 0.049ms) reported by a
user with ~77k observations. The array overlap operator (&&) on
source_memory_ids was doing a full sequential scan over all observations,
causing recall timeouts (57-64s) and slow user recall (18-27s avg).
The partial GIN index reduces consolidation recall from timeout to ~15s
and user recall to ~6s.
* fix: use pre-bounded memory_links for observation graph expansion
Replace raw unit_entities join in _expand_observations() with the same
memory_links entity graph used by non-observation fact types. The previous
approach joined unit_entities twice (seeds→entities→connected_sources),
which explodes at scale (30-70s at 100k observations). The LIMIT 500
workaround was non-deterministic and dropped valid results.
Using memory_links (pre-bounded to MAX_LINKS_PER_ENTITY=50 at retain time)
is algorithmically identical to the non-observation entity expansion and
keeps graph retrieval at ~2s p50 even at 100k observations.
Also fix migration down_revision (z1u2v3w4x5y6 → d2e3f4a5b6c7) and add
observation generation + fact-type filtering to the recall perf benchmark.
ad2cf72aab