Who I am comes from signing in; my profile is the document I own

The app derived its identity from a profile lookup and, when nothing matched,
picked somebody else. That is backwards: signing in returns who I am, and the
profile is looked up by it.

- Identity and profile are now two things. The identity is what
  `ensureIdentity()` returns: opaque, never rendered, never written, never
  passed to a data-layer call. The profile is Festipod's own object -- pseudo,
  name, initials -- in a document we create and write.
- "My profile" is the profile document I own, resolved through
  `listMyEntityDocs('protected')`. No username matching, no positional pick. A
  failed listing leaves the answer UNKNOWN rather than collapsing to "none".
- Having no profile now resolves to having no profile. Two impersonation
  fallbacks are gone, including one in `updateProfile` that would have written
  your pseudo into a stranger's document.
- A profile is created at sign-in when none exists. The shape makes name,
  initials and username mandatory, so it is written with placeholders that read
  as instructions -- never a plausible human name, never anything derived from
  the opaque identity.

Nothing succeeds in silence any more
`joinEvent` used to return without writing and without throwing when it could
not attribute the participation, while the screen announced success. It rejects
now, and the confirmation follows the write. Withdrawal likewise -- the doctrine
requires it to be authoritative. The host notification stops being written into
the joiner's own store, where its recipient could never read it, and the
optimistic notice shown to the wrong person goes with it.

The creator signs up through the common path: no owner branch anywhere, no
special case, the same deposit and the same derived count.

.project/concepts/app-architecture/_debt.md

@@ -0,0 +1,9 @@
+# Doc-debt — app-architecture
+
+> Presence of a block = doc to update. Processed → delete the block; no blocks left → delete this file.
+> One block = one "big change": `why` + `files` + `verify` (leaves to review).
+
+## Raw markers (consolidate into blocks, then delete)
+- TOUCHED src/shared/context/FestipodDataContext.tsx @2026-08-16 (session 0b064e8b-1717-421f-a20e-a4318ad217b1)
+- TOUCHED src/modules/event/screens/EventDetailScreen.tsx @2026-08-16 (session 0b064e8b-1717-421f-a20e-a4318ad217b1)
+- TOUCHED src/modules/user/screens/UpdateProfileScreen.tsx @2026-08-16 (session 0b064e8b-1717-421f-a20e-a4318ad217b1)