Configurable festival "day end" hour so post-midnight sets group with the previous night

[chiptus]

Problem

Festival sets often run past midnight. Once the Schedule groups sets by the festival timezone calendar day (#75), a set that starts at 01:00 on Saturday falls under the "Saturday" calendar day — but festival-goers think of a 01:00 set as part of Friday night. Grouping strictly on the midnight boundary splits a single night across two day headers and can hide late sets under a day the attendee doesn't expect.

Desired direction

Let a festival define a day end / day start hour (a cutoff, e.g. 06:00) so the "festival day" runs from that hour to the same hour the next calendar day. Everything before the cutoff groups and labels under the previous festival day. Sets from 20:00 Friday through 05:00 Saturday would all read as "Friday".

Context / prior decisions

• Deferred during the festival-timezone grilling session (Q6). Recorded as out-of-scope in the parent PRD Show schedule times in the festival timezone, not the viewer's #74 and in docs/adr/0002-festival-timezone-display.md ("A custom per-festival day-start hour was considered and deferred as a separate feature").
• Depends on Festival timezone: schema, admin config, and Schedule list view in festival time #75, which establishes festival-timezone calendar-day grouping as the baseline this cutoff modifies.
• The day-grouping logic being modified is the pure day-key/day-label helper introduced in Festival timezone: schema, admin config, and Schedule list view in festival time #75 (single seam in src/lib/timeUtils.ts); this feature widens that helper to take a cutoff hour. The horizontal timeline's day boundaries (Horizontal timeline Schedule view in festival time #76) would need the same treatment.

Open questions (resolve before this is agent-ready)

• Where does the cutoff live — a new column on festivals (consistent with how timezone is stored), or a global constant?
• Default value if configurable (06:00? 05:00?), and is it required/nullable.
• Admin UI to set it (likely the festival edit dialog, alongside timezone).
• Scope: does the cutoff affect only day grouping + labels, or also the day filter and the morning/afternoon/evening buckets? Does it apply to the horizontal timeline day boundaries too (Horizontal timeline Schedule view in festival time #76)?
• Interaction with the schedule reveal level days display (the "which day" a set plays).

Suggested next step

Run /grill-with-docs then /to-prd to lock the open questions and produce an agent-ready spec before implementation. Do not start implementation until #75 has landed.

[chiptus]

This was generated by AI during triage.

Triage: enhancement + ready-for-agent

Grilled during triage. Blocker (festival-timezone day grouping) has shipped, and the design is now fully pinned. The whole feature routes through one existing seam, which makes it a clean agent task.

Decisions locked

• Storage: a day_start_hour (integer hour, 0–23) column on festivals, NOT NULL DEFAULT 0, inherited by editions — mirroring how timezone is stored per ADR 0002 (a festival's day-start convention rarely changes between editions).
• Default 0 = opt-in: with the default, grouping is identical to today's midnight behavior; no existing schedule silently re-groups on deploy. A festival sets e.g. 6 to fold post-midnight sets into the previous night.
• Definition of "festival day": a set's festival day is the festival-timezone calendar date of its start instant shifted back by day_start_hour hours. Equivalently, sets from day_start_hour:00 on day D through just before day_start_hour:00 on D+1 all belong to festival-day D, and the day header reads D's date. (Cutoff 0 ⇒ today's exact behavior.)

Scope — where the cutoff applies

Widen the single festival day-key helper (getFestivalDayKey / getFestivalDayLabel) to take the cutoff and thread the festival's day_start_hour through the consumers. That makes these consistent in one move:

• Schedule grouping (useScheduleData) and the day headers/labels.
• The day filter (options + matching) in both list and timeline.
• The days reveal-level "which day" display. (Confirmed feasible: maskSetForReveal keeps time_start at the days level, so the adjusted day is computable even when the time isn't shown.)
• The horizontal timeline day-divider boundaries and labels (their own layout calc needs the cutoff applied explicitly, not just via grouping).

Does NOT apply to the morning/afternoon/evening time-of-day buckets (getFestivalHour) — those are literal wall-clock time-of-day and stay unchanged; a 2am set is still "night" regardless of which festival-day it groups under.

Admin

Add a control to set day_start_hour in the festival edit dialog, alongside the timezone field (hour 0–23; surface the current value; a hint that 0 = split at midnight). Follow the existing react-hook-form pattern used there.

Verification

• Set a festival's day_start_hour to 6: a set starting ~02:00 moves under the previous festival day in both the list and the horizontal timeline; the day filter and the days-reveal day display agree; the day header reads the previous date.
• A festival left at the default 0 renders identically to before.
• pnpm run typecheck, pnpm run lint, pnpm exec vitest run, pnpm run build pass. Add/extend unit tests for the widened day-key helper (cutoff 0 parity + a non-zero cutoff around the boundary).

Docs

Record the decision — either a short new ADR or an amendment to docs/adr/0002-festival-timezone-display.md (which currently notes this feature was "considered and deferred").

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