Make EFT paste a pure sandbox: reviewable submissions require inventory validation

[TrueMessenger]

Summary

• The Test a Fit paste flow is now a pure sandbox for every user: pasting an EFT fit always grades it and shows findings, but never creates a FitSubmission, never notifies reviewers, and never reaches the review queue. Previously doctrine managers and reviewers could POST without mode=check_only to persist a real submission from a paste.
• The mode POST field is now vestigial - accepted (including from legacy/cached forms) but has no effect, since the sandbox path always runs.
• Re-checking a submission now only works for ESI-sourced submissions, since a re-check re-validates against the pilot's live inventory. Legacy EFT-paste submissions from before this change remain visible as read-only history; hitting re-check on one now shows an explanatory message and redirects back instead of silently no-op'ing against stale text.

Why

Pasted text is unverifiable - a pilot could paste the doctrine's own EFT export verbatim without owning the ship. Reviewable submissions should only ever come from something that was actually validated against a pilot's real inventory (ESI / corptools), so the paste tool is repositioned as what it's always been safest as: a self-check sandbox.

How tested

• manage.py test fitcheck - full suite green (717 tests).
• manage.py check - clean.
• manage.py makemigrations fitcheck --check --dry-run - no changes (no model/behavior requires a migration).
• Manually traced the mutated/abyssal-module follow-up flow (paste containing an abyssal module with no rolled stats) to confirm it still completes end-to-end as a sandbox result after entering manual stats.