fix(hub): require auth and distinct reporters for content reports

POST /v1/reports had no authentication and no rate limit, and counted
every raw report row toward AUTO_BLOCK_THRESHOLD regardless of who
sent it or from where — two anonymous requests naming any blake3 hash
added it to the hub-wide content blocklist. A network-wide censorship
and DoS primitive for anyone who learns a public file's hash.

- require a signed-in account (get_current_user)
- rate-limited (10/hour)
- threshold now counts DISTINCT reporting accounts (reporter_id),
  one vote per account per hash; raised 2 -> 3
- refused outright (403) when the hub has public groups switched off:
  a private-only hub brokers no public content and nothing syncs the
  blocklist, so the endpoint would be pure abuse surface
- admin blocklist management (/v1/admin/blocklist*) is untouched, so a
  manual block still works regardless of the public-groups setting

Noted while fixing: no node currently consumes ContentBlocklist
(swarm_register checks the separate CSAM list), so the network-wide
block effect was latent — the abuse surface (DB fill, poisoned
moderation signal) was live today.

Tests rewritten in test_moderation.py.

Third security review, finding H2.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011pG75yGK3NthNfyjH74omG

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