test: build the fixture config and the keyset with ecoretest

The hand-built ini in web_test.go, the random fernet key in authn's
TestMain and the same seeding copied into the git-hook test are one call
to ecoretest now. The keys are fixed rather than generated on purpose:
they secure nothing inside a test process, and a constant keyset is what
lets two packages of this service initialise without the second rotating
what the first sealed with.

The synthetic instance runs in production mode, so the environment
banner is off in tests unless one asks for it.

feat: auto-provision companion Dolt DBs from git.sr.ht pushes

Add a service-to-service path so pushing a git.sr.ht repo creates a matching
Dolt database at ~owner/name, ready before the user's first `dolt push`.

- web: POST /internal/repos, guarded by internal-IP + network-key `Internal`
  auth (not the browser cookie/CSRF). Resolves/mirrors the owner via
  auth.LookupUser, then CreateRepo + InitStore, rolling back the row if the
  store init fails. Idempotent: an existing companion returns 200, a fresh one
  201 — safe to call on every push.
- cmd/dolt-git-hook: the git.sr.ht post-update-script. Delegates every hook
  stage to the stock /usr/bin/git.sr.ht-update-hook unchanged (argv[0], stdin,
  env, exit code preserved; fail-closed if the delegate is missing), then on
  post-update POSTs the companion create and prints a one-time clone notice.
  Best-effort: never fails a push, degrades to a warning on any misconfig.

Tests cover the endpoint (provision/idempotent/rollback/bad-input) and the hook
(signed request round-trips through the guard's decryption, notice only on 201).