Enforce per-user ownership on survey media in Storage rules

[rfontanarosa]

Media authorization in storage/storage.rules is survey-scoped, while the equivalent Firestore rules are owner-scoped.

Storage rules authorize media at survey level, not owner level:

• Reads leak: allow read: if canViewSurvey() ignores canViewDataCollectedByOthers, and read includes list — any collector can list and download everyone's photos, even when Firestore hides their submissions.
• Writes can't be owner-scoped: Authorize user-media writes at the survey level #2576 had to fall back to organizer-only for update/delete, so owners can't manage their own media.

Cause: the upload path user-media/surveys/{surveyId}/submissions/{taskId}-{uuid}.jpg carries no user or submission ID, and putFile() sets no metadata. The owner is in Firestore (submission['5']) but only reachable submission → path; rules can't query.

Fix: add {submissionId} to the upload path (ground-android) so rules can do an exact firestore.get() and mirror firestore.rules:206-215. Needs a migration for existing objects.

[gino-m]

If you store images by user ({uid}) you can avoid doing another query to fetch the submission.