docs: add a restore procedure, fix the backup cadence, and correct quota_tolerance

Four things, all found by the same question: what does an operator standing at the
venue actually need?

A RESTORE PROCEDURE. There was none anywhere, and a backup you have never restored
isn't a backup. Two hazards worth writing down: media must be extracted preserving
ownership (the app runs as uid 100 / gid 101, and a root-owned restore makes every
upload fail with EACCES surfacing as a generic 500), and the app must be STOPPED
first, because migrations run on boot and a live pool will fight the restore.

Both the backup and the restore commands were run against the real stack before being
written down, which caught two that would have failed:

  - The plain `pg_dump` did not restore: `psql` aborted on `ERROR: schema
    "_sqlx_test" already exists`. pg_dump emits no DROPs without --clean --if-exists,
    so the documented dump could only ever be restored into an empty database. Fixed
    at the source (the dump is now self-cleaning) and verified end to end: 16 tables
    back, exit 0.
  - `--same-owner` does not exist in BusyBox tar, which is what `alpine` ships, so
    the extract aborted before unpacking anything. `--numeric-owner` plus the
    explicit chown, verified to land 100:101.

BACKUP CADENCE. "Weekly offsite" is the wrong shape when every irreplaceable byte is
created in one eight-hour window and nobody can retake a wedding. The backup that
matters runs that night, and again after the release so the keepsake is captured.
Also: take the DB dump and the media tarball back to back, or you get rows pointing
at files the dump doesn't know about.

quota_tolerance WAS DOCUMENTED AS SOMETHING IT ISN'T. .env.example called it "fraction
of disk that triggers the low-storage warning". It is the multiplier in
`floor(free_disk * tolerance / active_uploaders)` -- so an operator who wants "warn me
later" and sets 0.95 is actually authorising guests to fill 95% of the disk, moving
the fixed point from 43% to ~49% and eating the export headroom. The admin UI labelled
it "Toleranz (0-1)" with no explanation at all, which invites exactly that reading;
it is now "Speicher-Anteil für Gäste" with the formula in the hint. Wrong docs on a
tuning knob are worse than no docs.

SIZING. New section with the arithmetic: three volumes on one filesystem, the quota
fixed point at tolerance/(1+tolerance), and the fact the 80 GB baseline does not cover
the keepsake -- both archives are built concurrently and each is roughly a second copy
of every original. Provision ~3x expected media, or give exports its own volume.

Also ticks the low-disk alert off the roadmap, since it now exists.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

frontend/src/routes/admin/+page.svelte

@@ -105,7 +105,20 @@
 					kind: 'bool',
 					hint: 'Reserviert für künftige Anzahl-Limits.'
 				},
-				{ key: 'quota_tolerance', label: 'Toleranz (0–1)', kind: 'number' },
+				{
+					key: 'quota_tolerance',
+					label: 'Speicher-Anteil für Gäste (0–1)',
+					kind: 'number',
+					// "Toleranz (0–1)" with no hint invited exactly the wrong reading — that a higher
+					// number means "warn me later". It is the multiplier in
+					// `floor(freier Speicher × Anteil / aktive Uploader)`, so raising it authorises
+					// guests to fill MORE of the disk, not less.
+					hint:
+						'Anteil des freien Speichers, den alle Gäste zusammen belegen dürfen: ' +
+						'Limit = freier Speicher × Anteil ÷ aktive Uploader. Kein Warnschwellenwert — ' +
+						'ein höherer Wert gibt MEHR Speicher frei. Das Keepsake braucht zusätzlich ' +
+						'etwa das Doppelte der Mediengröße; 0,75 ist der getestete Standard.'
+				},
 				{ key: 'estimated_guest_count', label: 'Geschätzte Gästezahl', kind: 'number' }
 			]
 		},