Move session schedule from hardcoded ICS to cohorts table fields

[unforced]

Context

The ICS generator (src/lib/ics.ts) currently hardcodes the live-session schedule as Mondays 5:30–7:30pm America/Denver. This works for Cohort 1 but won't survive:

• A cohort that meets on a different day of the week
• A cohort in a different timezone
• A schedule change mid-cohort
• Future cohorts we run in parallel with different cadences

Proposal

Add these columns to cohorts:

• session_day_of_week (integer 0-6, or ISO 1-7; pick one and stick to it)
• session_start_time (text, HH:MM local in session_timezone, e.g. "17:30")
• session_duration_minutes (integer, default 120)
• session_timezone (text, IANA zone — e.g. "America/Denver")

Default to the Cohort 1 values during migration so behavior is unchanged.

Then update generateCohortICS() to read from the cohort record instead of module-level constants, and drop the SESSION_* constants. VTIMEZONE blocks need to be generated per timezone — either ship a small map of common zones, or require the admin to pick from a known list at cohort creation.

Scope

• Migration + schema update
• Admin UI: add schedule fields to the cohort edit form (doesn't exist yet — see the inline meeting-URL form on /admin)
• ICS generator reads from cohort instead of constants
• Homepage / cohort-page display copy reads from the same source (today it's hardcoded in 4–5 places — see the "Mondays 5:30–7:30pm MT" string)

Out of scope

Editing the schedule mid-cohort without breaking already-downloaded .ics files. That's the subscribable-feed issue.

[unforced]

Extending the scope of this issue: the physical venue is also hardcoded in the ICS generator now (Regen Hub — 1515 Walnut St, Suite 200, Boulder, CO). When the schedule fields migrate to the cohorts table, add these venue fields too:

• venue_name (text, e.g. "Regen Hub")
• venue_address (text, e.g. "1515 Walnut St, Suite 200, Boulder, CO 80302")

ICS generator already builds the LOCATION string as "{name} — {address}" — migrating to read from the cohort row is a one-line change.