Introduces second_brain.pipeline_settings — the single-row config row
the upcoming web dashboard edits and the workers read at the start of
each run. Pinned to id=1 by a CHECK constraint so upserts-by-PK keep
the table singleton, and the migration seeds the row with the table's
column defaults via INSERT ... ON CONFLICT DO NOTHING.
Two consumer groups carved out:
- transcription_* fields persist now; future tower-side worker reads them.
- extraction_* fields will be read by the existing scheduler in the next
commit, which is the actual behavior change Travis cares about today.
The settings_store helper centralises get/update + form-parsing
(time-of-day, int-or-none) and active-window math so the routes and the
scheduler don't reimplement them.
