[events] event tagging and reporting

apps/api/migrations/0011_report_event.sql

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+-- Reporting a player EVENT (`POST /api/playerevents/v1/report`) reuses the report
+-- table rather than getting one of its own: it is the same submission with the same
+-- fields (category, free-text details, the reporter from the token) and the same
+-- moderation life — a moderator acting on it sets `banned` on the row exactly as they
+-- would for a player report. Generated from src/reports-db.ts (SCHEMA_DDL) — keep in
+-- sync.
+--
+-- `event_id` names the reported event; NULL on every ordinary player report, which is
+-- what tells the two kinds apart. The row's other columns are still filled in from the
+-- event: `reported_player_id` is its CREATOR (the person a moderator would act
+-- against — the column is NOT NULL, and "who is answerable for this event" is the only
+-- honest answer), and `room_id` the room it runs in, read from the event table so the
+-- client doesn't have to send either.
+--
+-- Partial index: event reports are a small minority of rows, so indexing only the ones
+-- that name an event keeps "reports against this event" off a full scan without paying
+-- for the NULLs.
+
+ALTER TABLE report ADD COLUMN event_id INTEGER;
+CREATE INDEX IF NOT EXISTS idx_report_event ON report (event_id) WHERE event_id IS NOT NULL;