fix(automations): resolve friendly template fields ({user_name}, {challenge_title}, …)

[tbcsec]

Fixes #27

Root cause

Event payloads carry raw ids, and render_template substitutes payload values verbatim — so {user_id} in an announcement body rendered a UUID.

Fix — friendly companion fields

Before a matched rule's actions run, the engine resolves every id field in the payload into a human-friendly companion:

id field	friendly companion
user_id (+ opener/assignee/author/actor_user_id)	user_name (+ matching *_user_name)
team_id	team_name
challenge_id	challenge_title
survey_id	survey_title
ticket_id	ticket_subject
competition_id	competition_name

• Resolved once per rule run in run_rule — the single choke point both the engine and the time-based scheduler pass through, so every template-rendering action (notify / announcement / email / webhook body) sees them.
• The rule builder now suggests them: the catalog derives each trigger's friendly fields from the same map, so {user_name} & co. appear in the field suggestions automatically.
• Design call — ids stay ids: {user_id} still renders the id (webhook payloads and conditions legitimately need raw values). The friendly names are companions, not replacements — the builder's suggestions steer authors to the right placeholder.
• Unresolvable ids (entity deleted between event and rule-fire) fall back to the raw value, so an advertised placeholder never renders as a literal {user_name}. Enrichment is fail-open — a lookup error logs and the rule still runs.

Verification

• ✅ 3 new tests: enrich_payload unit (resolution, id fallback, input non-mutation), run_rule end-to-end (announcement body renders grace drew first blood on Warmup., not UUIDs), catalog advertising per trigger
• ✅ Full backend suite: 377 passed

Follow-up (after this + #31 merge)

The demo seed's first-blood announcement can then showcase this: "{user_name} drew first blood on {challenge_title}!" — which also makes successive demo announcements visibly distinct (the perception half of #19). Kept out of this PR to avoid a demo.py conflict with #31.

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