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/database/sql/schemas/download_items.sql

@@ -1,4 +1,4 @@
--- One row per grab attempt against one candidate.  A request can have
+-- One row per grab attempt against one candidate.  A download can have
 -- several: the first pick stalls, the user picks another, or a
 -- search-only provider's candidate is fetched by a separate transport
 -- (in which case provider_id is the searcher and transport_id is the
@@ -17,7 +17,7 @@
 
 CREATE TABLE IF NOT EXISTS download_items (
     id           TEXT PRIMARY KEY,
-    request_id   TEXT    NOT NULL,
+    download_id  TEXT    NOT NULL,
     provider_id  INTEGER NOT NULL,
     transport_id INTEGER,
     external_id  TEXT    NOT NULL DEFAULT '',
@@ -35,15 +35,18 @@ CREATE TABLE IF NOT EXISTS download_items (
     error        TEXT    NOT NULL DEFAULT '',
     created_at   DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
     updated_at   DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
-    FOREIGN KEY(request_id) REFERENCES download_requests(id) ON DELETE CASCADE
+    FOREIGN KEY(download_id) REFERENCES download_downloads(id) ON DELETE CASCADE
 );
 
 CREATE INDEX IF NOT EXISTS idx_download_items_live
     ON download_items(state)
     WHERE state NOT IN ('complete', 'cancelled', 'failed');
 
-CREATE INDEX IF NOT EXISTS idx_download_items_request
-    ON download_items(request_id);
-
 CREATE INDEX IF NOT EXISTS idx_download_items_state
     ON download_items(state);
+
+-- idx_download_items_download is deliberately NOT declared here: on an
+-- existing database this table already exists at schema-pass time with
+-- its old column still named request_id, so an inline CREATE INDEX on
+-- download_id would fail outright. See ensureDownloadIndexes in
+-- backend/database/download_rename_migration.go.