Auto-fill polaroid captions from photo EXIF

[ankitsejwal]

Implements #4 — the headline feature. On upload, each photo's captions auto-fill from its EXIF: city (top) from GPS via the offline reverse-geocoder (#5), date (bottom, e.g. "October 2008") from capture time. Runs async after the photo appears; never overwrites a caption you've typed.

• lib/exif.ts — reads GPS + capture date via exifr (client-side)
• lib/date.ts — formats the date as "Month Year"
• store enrich() — EXIF → reverse-geocode → fill empty captions

Verification (real GPS photos, app's actual deps)

DSCN0010.jpg => captionTop: "Cortona" | captionBottom: "October 2008"
DSCN0021.jpg => captionTop: "Cortona" | captionBottom: "October 2008"
DSCN0027.jpg => captionTop: "Cortona" | captionBottom: "October 2008"

pnpm lint/typecheck/build all green.

Closes #4