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/ipc/api.go

@@ -176,6 +176,51 @@ type IngestMailResp struct {
 	Skipped bool    `json:"skipped,omitempty"`
 }
 
+// DescribeImageReq — one image handed to core to look at (Vikunja #252).
+//
+// Data is the raw image file as received (png / jpeg / gif). Core sniffs it and
+// refuses anything else; a declared content type is not part of this request
+// because the sender's claim about its own bytes is not evidence. Base64 on the
+// wire via the usual JSON marshal of []byte.
+//
+// Question is what he asked about the picture ("что тут написано?"). Empty ⇒
+// core uses its configured default prompt.
+//
+// Source is provenance recorded on the stored blob: "telegram", "web:upload".
+//
+// Exactly one of Data or ID is set. ID re-describes an image core already has —
+// a different question, or the first attempt that succeeds after a vision model
+// finally lands on disk.
+//
+// The method exists only when core has both a media store and an enabled vision
+// block; otherwise it answers ErrUnknownMethod, which is what "off unless
+// configured" looks like at the wire. A surface cannot make Maven look at
+// pictures by merely sending one.
+type DescribeImageReq struct {
+	Data     []byte `json:"data,omitempty"`
+	ID       string `json:"id,omitempty"`
+	Source   string `json:"source,omitempty"`
+	Question string `json:"question,omitempty"`
+	// SaveNote — also write the description as a note (source
+	// "media:image:<id-prefix>") so it is recallable later. Default false: a
+	// glance at a screenshot is not automatically a memory.
+	SaveNote bool `json:"save_note,omitempty"`
+}
+
+// DescribeImageResp — what she saw. ID is the stored blob's content address, and
+// it is set even when Description is empty because the description failed: the
+// bytes are on disk and the same id can be retried. NoteID is non-zero only when
+// SaveNote was set and the write succeeded.
+//
+// The image itself is never echoed back.
+type DescribeImageResp struct {
+	ID          string `json:"id"`
+	Description string `json:"description,omitempty"`
+	Width       int    `json:"width,omitempty"`
+	Height      int    `json:"height,omitempty"`
+	NoteID      int64  `json:"note_id,omitempty"`
+}
+
 // 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