log: replace logrus with slog behind auxilia's scribe handler

Every logger field this service owned was a *logrus.Entry threaded
through a constructor, which is what logrus costs for want of a usable
default. They are slog.Default().With("component", ...) now, and the
threading is gone with them; the shared middleware's panic reports land
in the same handler, which is why the daemon sets the default before
anything that can fail.

The handler is scribe's tint handler: level from [dolt.sr.ht]log-level,
source positions, and masks keyed on the attribute path for the three
credentials this service handles — the unified-login cookie, the
Internal fernet token and the Authorization header the remotesapi reads
a PAT or a keypair JWT out of. Errors go through scribe.Err, so a culpa
error's hint reaches the operator on its own line.

logrus stays in go.mod: dolt's remotesrv.ServerArgs takes a
*logrus.Entry and nothing else. It is now confined to Config.DoltLogger,
which is the only place this service names it.

dolt-git-hook is deliberately untouched: what it writes to stderr is the
notice a pushing user reads through git, not a log.

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.

rename module to sourcecraft.dev/bigbes/sr-ht-dolt; depend on sourcecraft sr-ht-core

remoteapi: authz interceptors, remotesrv assembly, credentials service

authn: cookie, PAT, and dolt-JWT caller resolution

Add the authn package resolving the SourceHut caller across dolt.sr.ht's
three auth flows, producing core-go *auth.AuthContext values mapped onto the
pure core.Caller domain type:

- ctx.go: WithCaller/CallerFromContext (nil for anonymous, never panics) and
  AsCoreCaller (maps UserType, derives Suspended).
- cookie.go: OptionalCookieMiddleware, the never-rejecting unified-login
  cookie reader (fernet decrypt -> {name} -> LookupUser); any failure degrades
  to anonymous so public browsing keeps working.
- token.go: ResolveBasic, the meta personal-access-token trio (offline
  DecodeBearerToken -> username match -> LookupUser + revocation) with a 60s
  positive cache keyed by sha512(password); TokenGrantsAllow gates
  dolt.sr.ht/repos RO/RW grants (empty grants pass).
- jwt.go: ResolveDoltJWT, EdDSA JWS verification for dolt keypair auth (kid ->
  KeyStore pubkey, alg=EdDSA, aud/exp/sub checked, iss ignored), reusing
  dolt's creds.PubKeyToKIDStr for the kid<->pubkey integrity check.
- grpc.go: ResolveGRPCAuth dispatching Basic/Bearer/anonymous.

Meta lookup + revocation sit behind the MetaBackend interface and JWT keys
behind the KeyStore interface (implemented later by db/), so tests forge
cookies (fernet), PATs (BearerToken.Encode) and real Ed25519 JWTs against
in-memory stubs with no network or Postgres.