refactor(download): rename Want/Request to Request/Download, unify downloads flow, add auto-download guardrails

The durable "I asked for this" record was called Want, and the one-shot
search-and-grab attempt was called Request — names that didn't match
what either actually did. Want is now Request, and the old Request/Item
is now Download/DownloadItem, with a table-rename migration
(download_wants -> download_requests, old download_requests ->
download_downloads) safe against both fresh installs and existing data.

Every anchored manual download now upserts/reuses a durable Request
before running, so a "download now" that finds nothing is picked up by
the background reconciler automatically instead of just failing with
no trace — the gap that caused this session's repeated "no candidates
found" failures on the same album.

Also adds auto-download guardrails (file-size min/max with a preferred
target, allowed file types) that gate what the pipeline may grab
unattended, live-editable from a new settings section. The frontend's
wanted-view becomes downloads-view, with a new Downloads tab showing
attempt/transfer history that previously had no UI at all.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Y2Agd9af5hE7qzti2ackiS

backend/datamap/datamap.go

@@ -142,10 +142,17 @@ var tables = []Table{
 			"only; the sized variants beside it are derived filenames and " +
 			"must be expanded when deleting (see library.coverArtFileSet).",
 	},
+	{
+		Name: "download_downloads", Kind: Authored, Lifetime: Cascade,
+		Note: "One row per 'go find me this', i.e. one search-and-grab " +
+			"attempt. Cascades from libraries, and cascades onward to " +
+			"download_items. Terminal rows are history the user clears " +
+			"explicitly.",
+	},
 	{
 		Name: "download_items", Kind: Authored, Lifetime: Cascade,
 		Note: "One grab attempt per row, with the ranked candidate stored " +
-			"as JSON. Cascades from download_requests. The candidate blob " +
+			"as JSON. Cascades from download_downloads. The candidate blob " +
 			"is kept rather than re-derived because a provider's result " +
 			"set is ephemeral — the peer that had the files may be gone, " +
 			"and the row still has to explain why it was chosen.",
@@ -159,18 +166,13 @@ var tables = []Table{
 	},
 	{
 		Name: "download_requests", Kind: Authored, Lifetime: Cascade,
-		Note: "One row per 'go find me this'. Cascades from libraries, " +
-			"and cascades onward to download_items. Terminal rows are " +
-			"history the user clears explicitly.",
-	},
-	{
-		Name: "download_wants", Kind: Authored, Lifetime: Cascade,
-		Note: "The wanted list: one MBID per row, plus retry bookkeeping. " +
-			"Cascades from libraries, and from a parent artist want to " +
-			"the album wants it derived. Unlike download_requests these " +
-			"are not history — a want outlives every attempt made on it " +
-			"and is only removed by the user or by the library coming " +
-			"to own what it names.",
+		Note: "The durable request list: one MBID per row, plus retry " +
+			"bookkeeping. Cascades from libraries, and from a parent " +
+			"artist request to the album requests it derived. Unlike " +
+			"download_downloads these are not history — a request " +
+			"outlives every download attempt made on it and is only " +
+			"removed by the user or by the library coming to own what it " +
+			"names.",
 	},
 	{
 		Name: "explore_champion_fts", Kind: Cache, Lifetime: Retained, FTS: true,