fix: allow OAuth2 sign-up on team_members via @request.context rule (#22)

Every first Microsoft login failed with 403 "Only superusers can perform
this action": PocketBase applies the collection createRule to the
automatic record creation during OAuth2 sign-up, and team_members was
created with createRule null (superuser-only) on the wrong assumption
that the OAuth2 flow bypasses it. Since the pre-Azure records were
deliberately dropped, every user was a first login and nobody could get
in. Reproduced locally against a mock OIDC provider (PocketBase v0.30.4).

createRule becomes '@request.context = "oauth2"': record creation is
allowed exclusively from within the OAuth2 flow. Anonymous REST creates
(which could otherwise pre-seed rogue admin profiles) remain rejected —
covered by an explicit test.

- pb_migrations/1781000002_allow_oauth2_signup.js: applies the rule on
  already-migrated environments (Labs); idempotent, guarded, with down
- pb_migrations/1781000000_team_members_to_auth.js: same rule for fresh
  environments (filename unchanged — ledger-safe)
- scripts/setup-pb-collections.mjs: fallback entry mirrored
- docs/auth-spec.md: ADR 009 + files table

Verified with a mock-OIDC matrix (9/9): baseline reproduces the 403 on
the old rule; upgrade path heals (first login 200, role/enrollment/name
defaults, second login no duplicate, anonymous create still rejected);
fresh chain + admin allow-list mapping OK. DoD harness regression 15/15,
npm test 112/112.

Closes #22

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

pb_migrations/1781000000_team_members_to_auth.js

@@ -142,10 +142,14 @@ migrate((app) => {
     // Access rules: every legitimate user is authenticated (Azure-gated +
     // OIDC). Profiles are readable by any authenticated user (leaderboard,
     // dashboard); a user may only update their own record (onboarding flips
-    // enrollment_status). Creation/deletion happen via OAuth2 / superuser only.
+    // enrollment_status). Record creation is ONLY allowed from within the
+    // OAuth2 sign-up flow: PocketBase applies the createRule to the automatic
+    // record creation on a first OIDC login, so `null` would make every first
+    // login fail with 403 "Only superusers can perform this action" (issue
+    // #22). Plain REST creates keep being rejected for non-superusers.
     listRule:   '@request.auth.id != ""',
     viewRule:   '@request.auth.id != ""',
-    createRule: null,
+    createRule: '@request.context = "oauth2"',
     updateRule: '@request.auth.id = id',
     deleteRule: null,
     authRule:   "",