feat(scripts): seed v2 helpers and realistic-day scenario

[NC1107]

Summary

Adds a seed-v2 library and two scenario scripts that exercise gaps the existing seeders miss. The existing seed_demo_data.sh, seed_full_demo.sh, and seed_stress_data.sh are unchanged.

What ships

• scripts/seed_v2_lib.sh — sourceable helpers, REST + WS only (no DB access). Functions: seed_user, seed_login, seed_contact_accept, seed_group (with is_encrypted toggle), seed_group_invite_accept, seed_dm_message, seed_group_message (with reply_to_id), seed_reaction, seed_set_presence, weighted_choice, random_message_text, backdate_seconds, seed_log. Cloudflare-safe UA, set -euo pipefail, error trap that prints the failing line.
• scripts/seed_realistic_day.sh — 6 users with varied bios/presence, all-to-all contacts, one plaintext + one encrypted group (is_encrypted=true), all members join both, pairwise DMs (4-10 msgs each), and a realistic-day group rhythm: 08:00 standup, 09:30 async chat with branching reply threads (replies hit several different parents instead of all chaining to the first message), 12:00 lunch lull, 14:00 decision thread with 4 replies, 17:30 EOD. Plus ~15 reactions sprinkled across group messages.
• scripts/seed_dms_only.sh — minimal pure-DM fixture: 4 users, 6 pairwise DM threads of 8-15 messages each. No groups.
• One-paragraph mention in CLAUDE.md's Build & Run section so the scripts are discoverable.

Scenarios covered

• DMs (gap: existing seeders only do groups)
• Encrypted groups with is_encrypted=true so the [Encrypted] placeholder + lock indicator render. Posts ~120-char random-base64 strings as ciphertext placeholders since the Signal Protocol can't be driven from bash (file comment explains this).
• Conversation branching — replies hit multiple parents, not just one root.
• Presence variety — online, away, dnd, invisible (the server enum has no literal offline; the lib maps offline → invisible).
• Realistic message rhythm across the simulated day instead of one firehose burst.
• Idempotency-ish reruns via timestamp-prefixed usernames (riley_1716345678).

Gaps remaining

• Multi-device — no programmatic way to provision a second device key bundle from bash. Future work.
• Voice / video calls — LiveKit signaling is interactive and not covered.
• Push tokens — no APNs/FCM seeding (requires real device tokens).
• Backdated created_at — the server stamps messages.created_at itself; the lib's backdate_seconds is annotation-only. Truly historical history needs DB access.
• Disappearing-message TTL variants — endpoint exists (PUT /api/conversations/:id/disappearing) but no scenario uses it yet. Easy follow-up.
• @mentions / @here / @everyone — server stores them as message text + extracts via the parsing pass; the new scripts don't drive that yet. Easy follow-up.

The motivator was an audit of what the current seeders don't exercise — the gaps above are documented inline in the lib so the next pass knows where to extend.

Test plan

• Local: cargo run -p echo-server then SEED_SERVER=http://localhost:8080 ./scripts/seed_realistic_day.sh — verify 6 users land in the sidebar with varied presence dots, plaintext group has the standup/async/decision/EOD rhythm, encrypted group shows the lock indicator + [Encrypted] placeholders.
• Local: ./scripts/seed_dms_only.sh — verify 4 users, 6 DM threads, no groups.
• bash -n on all three scripts passes (verified pre-merge).
• Existing seeders still work: spot-check ./scripts/seed_demo_data.sh.