Store and describe images through a shared media intake (#252)

Vision needs a second model this box does not have, so the shipped half is
the part that works without one: an image arrives, is sniffed, is stored
content-addressed, and is prepared for inference. The describing half is
written and tested against a fake server, and refuses any endpoint that is
not on this box.

internal/media is the intake all three senses share — hearing and speaker
recognition store their audio in the same place under the same retention.
Blobs stay out of the sqlite store; only the derived text becomes a note,
and only when the caller asks. Retention is enforced by an hourly prune
loop rather than by a comment.

The plan's RemoteProvider step is refused: no cloud model, inference stays
on the box, and vision.NewLocal validates that at construction.

internal/auth/policy.go

@@ -87,6 +87,14 @@ func Requirement(m ipc.Method) Authority {
 		// reminder, or touch the tool allowlist, so a compromised mail reader can
 		// at worst put junk on a review page he clears in one click.
 		ipc.MethodIngestMail,
+		// Looking at one image (Vikunja #252). AuthRead because of what it can
+		// produce: words about a picture, and optionally a note. It cannot write
+		// a fact, set a reminder, or touch the tool allowlist. The invasive part
+		// of this capability is not the authority rung — it is that the bytes are
+		// kept on disk, which media.retention bounds, and that they never leave
+		// the box, which internal/vision enforces by refusing a non-private
+		// endpoint.
+		ipc.MethodDescribeImage,
 		// The read side of the model swap: which model is resident, which ones are
 		// allowlisted. It loads nothing and changes nothing.
 		ipc.MethodModelStatus: