Add link previews and icons; make PIG_INVITE_CODE actually work

Social and icons. A 1200x630 card, apple-touch-icon, and maskable PWA icons,
generated from an HTML template by a script so the mark, wordmark and tagline
cannot drift from the product. The apple-touch-icon referenced in index.html
was a 404 until now. Icons are drawn on an opaque plate with inset because iOS
rounds corners and Android may apply a circle — an edge-to-edge mark loses its
ears to that crop. og:image is absolute, which is the single most common
reason a card unfurls blank.

PIG_INVITE_CODE was a lie. Signup validates against the invites table, so
setting the variable only flipped a label in the UI — an operator would set it,
hand the code to a colleague, and watch them be rejected. It is now reconciled
into a real invite row at boot: setting it issues, changing it rotates and
revokes the predecessor, and removing it revokes. Verified all three, plus that
a restart with an unchanged code does not duplicate.

Two bugs found while doing that:

- `uses_remaining` was jsonb, so the SQL decrement could never have worked.
  Now integer. The generated migration failed because Postgres has no implicit
  jsonb->integer cast, so the USING clause is hand-written.
- Redemption keyed off `redeemedAt`, which would have made every reusable
  invite single-use — a confusing way to lock a team out. Availability now
  comes from `usesRemaining`, and redemption records who used it most recently
  without consuming it.

Sign-in gains a password option alongside the magic link, defaulting to
password since that is the daily path. PIG stores neither; both are handled by
the identity provider and PIG only ever sees the resulting token.
autocomplete is set so password managers and iOS can fill.

Verified: full migration chain applies to a fresh Postgres, the CSP hash for
the inline theme script is unchanged by the rebuild.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

apps/web/public/manifest.webmanifest

@@ -3,10 +3,15 @@
   "short_name": "pig",
   "description": "An agent-native CRM for two-sided AI-compute companies.",
   "start_url": "/",
+  "scope": "/",
   "display": "standalone",
+  "orientation": "any",
   "background_color": "#ffffff",
   "theme_color": "#ffffff",
   "icons": [
-    { "src": "/pig.svg", "sizes": "any", "type": "image/svg+xml", "purpose": "any" }
+    { "src": "/pig.svg", "sizes": "any", "type": "image/svg+xml", "purpose": "any" },
+    { "src": "/icon-192.png", "sizes": "192x192", "type": "image/png", "purpose": "any" },
+    { "src": "/icon-512.png", "sizes": "512x512", "type": "image/png", "purpose": "any" },
+    { "src": "/icon-maskable-512.png", "sizes": "512x512", "type": "image/png", "purpose": "maskable" }
   ]
 }