* feat: support timestamp="unset" to retain content without a date
When callers retain timeless content (e.g. fictional documents, static
reference material), passing timestamp="unset" now skips the utcnow()
default so mentioned_at is stored as NULL instead of an artificial date.
- HTTP: validate_timestamp recognises "unset" sentinel and threads it
through api_retain as event_date=None (key present, value None), which
the orchestrator distinguishes from key-absent (still defaults to now)
- Orchestrator: new branching logic separates "key absent" → utcnow()
from "key present but None" → no date
- types.py: RetainContent.event_date and ProcessedFact.mentioned_at are
now datetime | None; removed the unused _now_utc factory
- fact_extraction.py: all event_date params accept datetime | None;
_build_user_message emits "Event Date: Unknown" when None; removed
mentioned_at from the Fact LLM response model (LLM never sets it)
- embedding_processing: skip date suffix when fact_date is None
- entity_resolver: COALESCE(event_date, now()) for first_seen/last_seen
so entities table NOT NULL constraint is preserved
- link_utils: skip temporal linking for units without event_date
- Migration aa2b3c4d5e6f: DROP NOT NULL on memory_units.event_date
- Tests: test_retain_no_timestamp and test_retain_omit_timestamp_defaults_to_now
- Docs + OpenAPI + TypeScript client updated
* refactor: replace _TIMESTAMP_UNKNOWN sentinel with plain string comparison
The sentinel object() was only needed to distinguish "unset" from None
at the boundary — but since the field type is datetime | str | None,
"unset" can pass through the validator unchanged and be compared directly.
* chore: regenerate OpenAPI spec and clients after timestamp type change
timestamp field is now datetime | str | None to accept the "unset" sentinel value.
