Ship voice enrolment, and report recognition as blocked (#255)

Maven can now be told who someone is. She cannot yet tell who is speaking,
and this commit is careful to say so rather than pretend otherwise.

What works: profiles are enrolled from several deliberately recorded samples,
listed, and deleted. They live in the existing memory_vectors table under a
"speaker:" id prefix, so there is no migration; what that needed was a wider
interface than memory.Store, hence memory.Catalog with ByPrefix and Delete.
Delete is the load-bearing half — a voiceprint someone asked to be rid of has
to actually go, and a search-only store cannot do that. InMemoryStore.Insert
became an upsert by id to match what the persistent store already did.

What does not work, and why it is not faked: there is no speaker-embedding
model on this box. Sixteen ggufs in /mnt/hdd1/llms, all text; no ECAPA, no
x-vector, no titanet, no wespeaker, no .onnx anywhere under /mnt/hdd1. So
newSpeakerEmbedder returns nil, internal/speaker falls back to
speaker.Disabled, Identify answers ErrDisabled, and the daemon logs which
half is off at startup. The plan's "simple MFCC + GMM" floor is refused in
the package comment: MFCC cosine distance detects channel and loudness as
much as voice, and a biometric that is confidently wrong writes false claims
about named people into his memory. A bad floor is worse than none here.

Refused as well, and the reason is in enroll.go's doc comment: the plan asked
for unknown speakers to be enrolled on first interaction with a TTS "кто
это?". There is no request shape in the protocol that could express that.
Taking a biometric of whoever walks past the microphone does it to guests who
are not party to the exchange, and a synthesised question into a room is not
consent from whoever answers.

Authority: enrolment is AuthStepUp, because it is a deliberate sit-down act
that writes a biometric of a named person and never something done by voice
mid-conversation. Deletion is one rung lower at AuthWrite, deliberately
inverting the usual pattern — getting rid of a biometric must never be the
harder half. Listing is AuthRead and never returns the vectors themselves.

Off unless configured: no speaker block means the three methods answer
ErrUnknownMethod, so a default box has no wire path that takes a voiceprint.

make build and make test pass.

Vikunja #255

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TrVSBKe3RFDF4fGYKWYQnX

internal/ipc/api.go

@@ -301,6 +301,51 @@ type CaptureStatusResp struct {
 	Bytes   int       `json:"bytes,omitempty"`
 }
 
+// EnrollSpeakerReq — register a voice (Vikunja #255).
+//
+// Samples are separate utterances recorded deliberately for this purpose, not
+// audio harvested from ordinary turns. internal/speaker requires several of
+// them totalling enough seconds, and refuses one long clip: a profile built
+// from a single sentence encodes that sentence as much as the person.
+//
+// There is no "enrol whoever just spoke" request shape, and that omission is
+// the point. Taking a biometric of a guest because they walked past the
+// microphone is not something a wire protocol should make easy.
+type EnrollSpeakerReq struct {
+	ID      string        `json:"id"`
+	Name    string        `json:"name,omitempty"`
+	Samples []audio.Audio `json:"samples"`
+}
+
+// Speaker — one enrolled voice as a surface sees it. The voiceprint itself is
+// never sent: a listing says who is enrolled, it does not hand out the
+// biometric.
+type Speaker struct {
+	ID       string    `json:"id"`
+	Name     string    `json:"name"`
+	Enrolled time.Time `json:"enrolled"`
+	Samples  int       `json:"samples"`
+}
+
+// EnrollSpeakerResp — the profile that was written.
+type EnrollSpeakerResp struct {
+	Speaker Speaker `json:"speaker"`
+}
+
+// ListSpeakersResp — who is enrolled, sorted by id. Enabled is false when no
+// embedding model is wired, which is this box's state: the profiles can be
+// listed and deleted, nothing can be recognised.
+type ListSpeakersResp struct {
+	Speakers []Speaker `json:"speakers"`
+	Enabled  bool      `json:"enabled"`
+}
+
+// ForgetSpeakerReq — delete one voiceprint. This is the request that must
+// always work; a biometric someone asked to be rid of has to actually go.
+type ForgetSpeakerReq struct {
+	ID string `json:"id"`
+}
+
 // SwapModelReq — load another resident model without restarting the daemon
 // (Vikunja #250). ModelPath must be one of the paths in phraser.swap_models;
 // anything else is ErrForbidden, and an unconfigured allowlist makes the whole