[Core] Replace optional 72h deadline with mandatory 7-day deadline

[realproject7]

Summary

Change storyline deadline from optional 72h checkbox to mandatory 7-day (168h) for all storylines.

Rationale

• 72h too short for organic participation with small user base
• Optional deadlines lead to dead stories that stagnate indefinitely
• 7 days balances casual participation with forward momentum

Current: optional 72h checkbox on create page

New: mandatory 168h deadline, no checkbox, always active

Scope

Frontend

• src/app/create/page.tsx — remove deadline checkbox
• src/components/DeadlineCountdown.tsx — DEADLINE_HOURS 72 -> 168
• Writer dashboard, story page, discover page — deadline always shown

Backend

• Storyline creation routes — remove deadline_enabled field, always set deadline
• Supabase schema — if deadline_enabled column exists, always true or remove
• Agent storyline creation — same

Smart contract

• Check if StoryFactory enforces deadline on-chain
• If yes: always pass 604800 seconds (7 days)

Constants

• Update DEADLINE_HOURS = 168 everywhere
• Search for hardcoded 72 related to deadline

Migration

• Existing storylines without deadline: retroactively apply 7-day from last plot
• Existing expired: no change

Acceptance Criteria

• DEADLINE_HOURS = 168 everywhere
• No deadline checkbox on create page
• All storylines have mandatory 7-day deadline
• DeadlineCountdown shows 7-day timer
• No code path allows storyline without deadline
• Existing storylines handled
• Contract interaction verified
• No references to 72h remain