Adds a separate API-token section under /users/settings (distinct from
the beacon-monitor token list, since API tokens grant full account
access). The plaintext is surfaced exactly once via flash on creation;
only the SHA-256 hash is persisted, so revocation is the only path back
if the user loses it.
Also fixes the openapi.yaml link on /docs/api: the relative path
resolved to /docs/openapi.yaml from a no-trailing-slash URL and 404'd.
Adds bearer-token authenticated REST API at /api/v1 covering every
action a non-admin user can perform on the website: contact + beacon
submission, beacon-monitor management, propagation queries, profile
read/update, and self-service API token issuance/revocation.
Security: SHA-256-hashed bearer tokens (mwp_ prefix, plaintext shown
once at creation), RFC 9457 problem+json error responses, RFC 9651
RateLimit-* headers backed by an ETS bucket (600/min per token,
60/min per anonymous IP, 30/min on /auth/tokens), private-contact
filtering by viewer.
Docs at docs/api/README.md (prose reference) and docs/api/openapi.yaml
(OpenAPI 3.1 spec covering every endpoint, response, and schema).
Tests: 124 new tests across schema, plug, error renderer, rate
limiter, fallback, and every controller. 16/17 API modules at 100%
line coverage; FallbackController at 87.5% (one defmodule line, an
Erlang-cover artifact for action_fallback-only modules).
