Let findings be dismissed as false positives or accepted risk

[0xCardinal]

Closes #22.

What it does

• Dismiss / Restore in the issue details panel, with an optional reason (up to 500 chars).
• Dismissed findings leave the graph colouring, counts, search and the node panel. The Results panel shows "N dismissed findings hidden", and the Findings table has a Show N dismissed toggle that lists them greyed out.
• A dismissal is remembered for the audited repository or action, so it applies to later audits of it and to runs stored before it was made. Findings from pasted-YAML audits can't be dismissed, since there's no target to remember them against.

How

• backend/dismissals.py: every issue gets a fingerprint: a hash of its node id, type, identifying fields and message, leaving out what shifts between runs (line_number, latest_version, days_old, commit_date, digits in the message). A finding mirrored onto a package or image node shares its source's fingerprint.
• DismissalStore keeps dismissals per target in data/dismissals.json. They are applied when an audit is built and whenever a stored analysis is read.
• GraphBuilder leaves dismissed issues out of issue_count, node severity and the statistics, and adds dismissed_issues. It can also rebuild itself from stored graph data.
• New endpoints: POST /api/analyses/{id}/dismissals, DELETE /api/analyses/{id}/dismissals/{fingerprint}, GET /api/dismissals?target=.

Worth knowing

• Tested on all 201 stored analyses: the fingerprint never merged two distinct findings. If the finding itself changes (e.g. its step is edited), a dismissal of it no longer applies.
• unfiltered_network_traffic sometimes reports the exact same finding twice on one workflow. It's now counted and listed once, so some existing analyses show two fewer findings.
• Storage is server-side for now. A repo-level ignore file for team-wide and CI use could build on the same fingerprints.

Testing

• 17 new backend tests (fingerprint stability, mirrors, stats, store, endpoints); the full suite passes (558 tests).
• Checked in the browser against a stored analysis: dismissing one finding took every count from 148 to 147 and applied to other stored runs of the repo; restoring brought everything back to 148.
• Usage page, API reference (regenerated) and AGENTS.md are updated.

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