fix(e12): close the eighteen findings the E12 review round left standing

The deep review of the branch raised 47 findings, 18 of which survived adversarial
verification. Grouped by what they actually were:

**The size ceiling inverted at its boundary.** `remaining_bytes` clamped to 0 for a user
already at their storage cap, and `build_ydl_opts` reads 0 as "no limit" — so the one
user over quota was the only one downloading with no ceiling at all. `check_enqueue`
could not catch it: it ran while the job was queued and the user was still under the cap.
The arithmetic moves into `quota.remaining_bytes` (where every other max_bytes decision
lives, and where it can be tested), and the worker now fails the job outright rather than
proceeding unbounded — as a non-retryable `quota_exceeded`, since retrying cannot help
until space is freed.

**Silent failures in the new account panel.** All three actions went through `quiet: true`
endpoints, which suppress the global error dialog on the promise that the caller reports
inline — with no `onError`, that promise was broken and a refusal rendered as nothing. On
the broken-SMTP instance this panel exists for, an admin would read silence as "the mail
went out". The co-invalidation set and the email-off predicate move to `lib/adminUsers`,
shared with the pending-access queue, so the two copies of these mutations cannot drift
apart again — which is how half of this was lost in the first place.

**A rollback recipe that destroyed data without working.** `alembic downgrade -1` steps
back one migration, but a release routinely ships several (0.80.0 shipped four): it drops
the newest one's columns and leaves the schema at a revision the older image also cannot
resolve, i.e. the crash loop it was meant to escape, minus the data. The dump restore is
now the recommended path, and the alembic route asks the target version for its own head
revision instead of counting backwards.

Also: redirects named in the urlguard docstring beside the DNS-rebinding residual they
share a fix with (SEC-17); a malformed port refused as `bad_port` instead of escaping as
a 500; the pull cooldown extended to the sibling route spending the same QuotaAction;
restore stops the app before dropping its tables; the PowerShell cleanup can no longer
decide the script's verdict on 5.1, nor report the wrong exit code; the documented
one-liners hardened to match the scripts; the registry's retention window stated where
pinning is advertised; the two banners made mutually exclusive so neither can shadow the
other; `hover:bg-hover` (a utility this project never generated) replaced by the
`selectable` recipe; the clipboard helper reused; and the sync cooldown's copy swept to
the two callers that discarded it.

Verified: gate green (vitest 678, pytest 949), and the panel, the row hover and the reset
banner UAT'd on localdev.

backend/app/downloads/urlguard.py

@@ -10,11 +10,19 @@ What this closes: a URL whose host resolves to a loopback / private / link-local
 is refused, at enqueue AND again in the worker before extraction. Both, because DNS can answer
 differently between the two moments, and the worker is where the socket is actually opened.
 
-What it does NOT close, stated plainly rather than implied: a host that passes the worker's check
-and then answers yt-dlp's own resolution with an internal address (DNS rebinding). Closing that
-needs the connection itself to be gated — a custom socket factory inside yt-dlp — which is a much
-larger change than this one. The two checks here stop the straightforward case (typing
-``http://127.0.0.1:6379`` or an internal hostname into the box), which is the reachable one.
+What it does NOT close, stated plainly rather than implied, because both need the same fix — gating
+the connection itself with a custom socket factory inside yt-dlp, a much larger change than this:
+
+* DNS rebinding: a host that passes the worker's check and then answers yt-dlp's own resolution
+  with an internal address.
+* HTTP redirects: a public host that answers 3xx with an internal ``Location``. yt-dlp follows it,
+  and only the URL the user TYPED was ever judged here. An attacker-controlled host or any public
+  open redirect reaches it, so this is the cheaper of the two — but it still buys only what a
+  media-shaped internal response yields, since a non-media body fails the mp4 remux and stores
+  nothing. Tracked as SEC-17.
+
+The two checks here stop the straightforward case (typing ``http://127.0.0.1:6379`` or an internal
+hostname into the box), which is the reachable one.
 
 Not used for YouTube jobs: those resolve to a fixed public host from an 11-char id, so there is no
 user-controlled destination to guard.
@@ -28,6 +36,7 @@ from urllib.parse import urlparse
 # vocabulary (frontend `localizeDetail`) — keep them stable.
 REASON_SCHEME = "scheme"  # not http(s)
 REASON_NO_HOST = "no_host"  # URL carries no hostname
+REASON_BAD_PORT = "bad_port"  # ":99999" / ":abc" — urlparse only raises on ACCESS, so guard it
 REASON_UNRESOLVABLE = "unresolvable"  # DNS said nothing (honest: not the same as "blocked")
 REASON_PRIVATE = "private"  # resolves into a network the server must not fetch from
 
@@ -79,7 +88,15 @@ def check_url(raw: str) -> None:
     host = u.hostname
     if not host:
         raise BlockedUrl(REASON_NO_HOST)
-    for addr in _resolve(host, u.port):
+    # `.port` is a lazily-parsed property: it raises ValueError for ":99999" or ":abc" on ACCESS,
+    # not at urlparse() time, and `.hostname` above happily returns first. Unguarded, one typo
+    # escapes the caller's `except BlockedUrl` as a 500 instead of the structured 422 this whole
+    # module exists to return.
+    try:
+        port = u.port
+    except ValueError as exc:
+        raise BlockedUrl(REASON_BAD_PORT) from exc
+    for addr in _resolve(host, port):
         try:
             ip = ipaddress.ip_address(addr)
         except ValueError:  # getaddrinfo returned something unparseable; treat as unusable