LEARN almanac entries (optionally filtered by month/day)

Return almanac entries as {entries: [...]}.

Behavior:

• Both month and day provided: returns entries for that calendar date (uses pg_get_learn_on_this_date_for).
• Neither provided: returns ALL entries (matches the pre-PR0 contract — app-renderers.ts:_renderOnThisDatePane fetches the whole file and filters client-side). The whole-table sweep is bounded (~35 rows today, hundreds at most) so we stream it back rather than forcing the caller to specify month/day they don't have at fetch time.
• Only one provided: ignored (returns full almanac) — narrower defensive than 400-ing; the caller's renderer already handles a too-broad result by filtering.

Falls back to the JSON file when PG is empty.