A second way in: signing in through an identity provider

OpenID Connect, off unless four settings are present. It is a second way to
the same token the password login mints — every route and the WebSocket
still know only a signed JWT — which is why it fits in one file and needs no
new dependency: Node imports a JWK itself, so checking an ID token is a
signature and five claims.

Authorization Code with PKCE, exchanged on the server so the client secret
stays there. The browser comes home with a one-time ticket rather than the
token, because a token in a redirect URL sits in the history, in the proxy's
log and in the referrer of the next request.

Somebody the app has not seen gets an account; an account that already
exists under that name is linked to the identity rather than duplicated, by
subject from then on. Never an administrator. An account made this way has
no password here, and the interface stops offering one — but an admin can
still set one, which is the way back in if the provider is ever gone.

The suite signs in at a provider of its own that signs real RS256 tokens,
and holds down the four ways the journey must fail as well as the way it
must work.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.env.example

@@ -31,3 +31,25 @@ INKNOTE_ADMIN_PASSWORD=
 # VERSION_RETENTION_DAYS=            # off: versions expire by count, not by age
 # VERSION_COALESCE_MINUTES=5
 # ALLOWED_ORIGINS=                  # only if the UI is on another host
+
+# ── Signing in through an identity provider (OpenID Connect) ──
+# Optional. Leave the four empty and the app is password-only; set all four and
+# the sign-in screen offers the provider as well. The password login stays in
+# either case — an unreachable provider must not lock anybody out of their own
+# notes. Somebody who signs in this way and has no account here yet gets one
+# (never an administrator: that is granted in the app, on purpose).
+#
+# In PocketID: a new OIDC client, redirect URI exactly
+#   https://<your-inknote-host>/api/auth/oidc/callback
+# The *server container* has to reach the issuer itself, not just your browser.
+#
+# The first sign-in binds an identity to an account of the same name here, so
+# this trusts the provider's usernames. Right for a provider whose names are set
+# by its own administrator; wrong for one where people pick their own.
+# INKNOTE_PUBLIC_URL=https://notes.example.com
+# INKNOTE_OIDC_ISSUER=https://id.example.com
+# INKNOTE_OIDC_CLIENT_ID=
+# INKNOTE_OIDC_CLIENT_SECRET=
+# INKNOTE_OIDC_LABEL=PocketID        # what the button says: "Continue with …"
+# INKNOTE_OIDC_SCOPES=openid profile email
+# INKNOTE_OIDC_REDIRECT_URI=         # only if it is not <public url>/api/auth/oidc/callback