fix: P0 hardening — fail-closed SECRET_KEY, locked CORS, role-safe registration (HARDENING.md P0.1/P0.2/P0.3)

P0.1 — fail-closed secrets:
- config.py: no default SECRET_KEY; refuses to boot when unset, a known
  placeholder, or <32 chars. Generate with: openssl rand -hex 32.
- docker-compose.yml: literal secrets removed; runtime env now comes from
  a git-ignored .env via env_file. .env.example added as template.
- .gitignore already covers .env (verified).

P0.2 — locked CORS:
- main.py: CORS_ORIGINS must be an explicit comma-separated allow-list.
  '*' or an empty value refuses to boot (was: silently ['*'] with
  allow_credentials=True).

P0.3 — role-safe registration:
- services/auth.py: client-supplied 'role' is IGNORED on POST
  /api/auth/register; self-registered users always get the
  least-privilege 'CS Rep' role. Unauthenticated callers can no longer
  mint Admin/Jerome, Admin/Wahab, or Director accounts.

Tests:
- conftest.py sets test SECRET_KEY/CORS_ORIGINS before app import.
- New tests/test_p0_hardening.py (8 tests): role-escalation blocked for
  Admin/Jerome and Admin/Wahab, duplicate-email 409, and subprocess
  boot-validation for placeholder/short/missing secret + wildcard CORS.
- Full suite: 44 passed.

Redeploy note (per research): seed_units/seed_categories are insert-only,
so the Aug-26 redeploy does NOT orphan historical tickets referencing
units 103E/103W/105E/105W or the legacy 34-category tree. Pending
Wahab: are 103E/103W/105E/105W real apartments dropped from the Excel
regeneration? Optional follow-up: floor-number backfill for already-
seeded units (mapping corrected floors; existing rows keep old values).

Checks per HARDENING.md acceptance:
- [x] starting without a real key fails loudly (subprocess-verified)
- [x] compose carries no literal secret; secrets come from .env
- [x] CORS_ORIGINS explicit allow-list, '*' rejected
- [x] unauthenticated register cannot mint Admin/* or Director

app/schemas/auth.py

@@ -10,7 +10,10 @@ class RegisterRequest(BaseModel):
     password: str
     full_name: str
     phone: str | None = None
-    role: str = "CS Rep"
+    # HARDENING.md P0.3: role is NOT client-controllable. Self-registration
+    # always creates the least-privilege role; privileged roles are assigned
+    # by an admin directly in the DB (or a future admin-gated endpoint).
+    role: str = "CS Rep"  # kept for backward compat; ignored by the service
 
 
 class LoginRequest(BaseModel):