TEN-WP-0003: FlexAuthWriteAuthorizer -- gate writes through flex-auth

flex_auth.py: CheckRequest + FlexAuthCheckClient against flex-auth's real
POST /v1/check contract (schemas/check_request.schema.json,
decision_envelope.schema.json, read directly from the flex-auth repo, not
guessed). Fail-closed by construction: only effect=="allow" authorizes;
every other effect, non-200, malformed body, or transport failure resolves
to deny, nothing raises past is_allowed().

authz.FlexAuthWriteAuthorizer implements the existing WriteAuthorizer
Protocol. Action -> resource-type mapping coordinated with FLEX-WP-0008's
planned vocabulary (both repos reference the same table).
DefaultDenyWriteAuthorizer stays the fallback when no flex-auth URL is
configured.

config.py: Settings.from_env(), mirroring qonto-assistant's pattern.
docs/flex-auth-integration.md documents the contract, fail-closed rule,
and current real state (denies everything until FLEX-WP-0008 lands).

60 tests passing. Verified live twice over real HTTP between separate
processes (not just MockTransport): a deny-returning flex-auth double
produces 403 from POST /tenants, an allow-returning one produces 201.

Also registered (not implemented) the two workplans this depends on for a
complete picture: flex-auth/FLEX-WP-0008 (protected-system registration --
what makes allow reachable) and key-cape/KEY-WP-0005 (discovered key-cape
emits none of iam-profile_v0.3.md's core claims yet, not just missing
tenant_roles -- a bigger, security-sensitive gap flagged rather than
quietly worked around).

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

INTENT.md

@@ -121,6 +121,8 @@ if the two drift, the canon contract wins and this file should be corrected.
 
 ## Related
 
+- `docs/flex-auth-integration.md` — how the write API's `WriteAuthorizer`
+  seam is implemented against a real `flex-auth`
 - `net-kingdom/canon/standards/tenant-engine-boundary-contract_v0.1.md` — the
   formal ownership contract
 - `net-kingdom/canon/standards/iam-profile_v0.3.md` — the `tenant_roles`