Expose live upload activity for shared files over EC (speed, active peers, last-active time) #466

Summary

The Web UI Shared Files page can only show the static counters the core already
exposes for a CKnownFile (transferred, requests, accepts, complete sources,
priority). Unlike the Downloads page, there is no way to see a shared file's
live upload activity: whether it is currently being uploaded, how fast, to
how many peers, or when it was last active.

This issue asks the core/EC layer to expose per-file upload activity for shared
files, so clients (Web UI, and later the desktop GUI) can add the corresponding
columns. Requests are grouped by implementation cost.

Motivation

Downloads already surface rich live state over EC (EC_TAG_PARTFILE_SPEED,
source counts, last_seen_complete, last_changed, active time). Shared files
have no equivalent, so a user cannot tell an actively-seeded file from an idle
one, or spot which shares are actually getting upload traffic. The underlying
data mostly already lives in the core objects (CFileStatistic,
m_ClientUploadList, CUpDownClient::GetUploadDatarate()); it is just not
summarized per-file or carried on the KNOWNFILE EC tags.

Current state

• Known-file EC tags: EC_TAG_KNOWNFILE_*, src/libs/ec/cpp/ECCodes.h:221-242.
  The set carries transferred (session/all), requests (session/all), accepts
  (session/all), AICH master hash, priority, complete-sources (+low/high),
  ON_QUEUE, comment, rating, media — but no upload speed and no
  active-uploader breakdown.
• Serializer: CEC_SharedFile_Tag, src/ECSpecialCoreTags.cpp:234-296 (base
  ctor shared with downloads). CEC_PartFile_Tag adds the download-only
  live-state tags (EC_TAG_PARTFILE_SPEED, source counts, last-seen).
• Core data that already exists but is not summarized per file:
  CKnownFile::m_ClientUploadList (set of clients uploading-to / queued-on the
  file), CKnownFile::GetQueuedCount(), CUpDownClient::GetUploadDatarate()
  (src/updownclient.h), CFileStatistic (src/KnownFile.h:60-87).
• Web API mapping: MergeSharedTag() in src/webapi/Refresher.cpp →
  FileSnapshot::SharedSide in src/webapi/State.h:199-222.

Requested fields

Tier 1 — data exists in core, needs a new getter + new EC tag

1. Current upload speed (per file) — sum of GetUploadDatarate() over the
   clients in CKnownFile::m_ClientUploadList. Analogous to
   EC_TAG_PARTFILE_SPEED. Enables a "Speed" column and an is-it-uploading
   indicator.
2. Peers uploading now — count of clients in m_ClientUploadList whose
   GetUploadState() == US_UPLOADING. Together with (1) this gives an
   upload-status signal ("uploading" vs "idle").
3. On-queue vs uploading split — today only the combined queue size is
   exposed (EC_TAG_KNOWNFILE_ON_QUEUE). Splitting "uploading now" from "waiting
   on queue" lets clients show uploading / queued the way Downloads show
   transferring / total sources.

Tier 2 — not currently tracked, needs a new persisted field

4. Last upload activity — timestamp of the last time data was sent for this
   file (or last request served). Note: CKnownFile::GetLastSeen() is
   "last on-disk match", not upload activity, so it is not a substitute.
5. Completed / shared-since timestamp — when the file finished downloading
   (or was added to shares). Lets clients show how long a file has been
   available.

Already available (no backend work — noted for completeness)

The following are already on the KNOWNFILE EC tags and can be surfaced by
clients without any core change; listed only so they are not duplicated as
requests: queued-client count (ON_QUEUE), share ratio (transferred / size),
and the complete-sources range (COMPLETE_SOURCES_LOW/HIGH).

Notes

• Tier 1 mirrors what already exists for part files, so it should be a contained
  change (new CKnownFile accessors + tags in CEC_SharedFile_Tag, decoded in
  src/webapi/Refresher.cpp).
• Tier 2 is heavier (new persisted state) and can be deferred; splitting it into
  a follow-up is fine.
• A companion idea proposes consuming these same fields in the desktop aMule and
  amulegui shared-files view; it depends on this issue landing first.