Passport town stamps: mark every CORE-authenticated town visit

[harshithmullapudi]

What

Every time a CORE-authenticated user visits a town, that town gets stamped onto their passport. The passport accumulates a visible collection of stamps (like a real passport) showing every town they've been to.

Why

• Turns the Passport into a living collection — every visit leaves a permanent mark.
• Creates a social / brag surface: "look at all the towns I've been to."
• Gentle nudge toward CORE login — stamps are gated on auth, so guests don't earn them.
• Feeds discovery: stamps on friends' passports become recommendations.

Gating

• CORE-authenticated only. Guest visits do not earn stamps. First-ever visit surfaces the payoff ("sign in with CORE to start collecting stamps").
• One stamp per town per user (idempotent). First visit stamps; re-visits update `lastVisitedAt` + `visitCount`.

Rough scope

• New model: `PassportStamp` (userId, townSlug, firstVisitedAt, lastVisitedAt, visitCount).
• Hook into existing visit tracking (TownActivity) — write a stamp on first entry for authed users.
• Passport page renders a stamp gallery (visual grid of town thumb + name + date).
• Stamp count on the passport header.
• Optional (later): custom stamp design per town — creator uploads / generates a stamp sprite; falls back to auto-generated.

Dependencies

• Depends on Passport (Passport: persistent, shareable player identity across towns #50) — needs the passport page to render stamps on.

Notes

• Don't stamp the creator's visits to their own town by default (avoid inflating their own count).
• Consider a rarity signal on stamps (small niche town vs. big public town) so a stamp on a hidden gem feels like a badge, not filler.