feat(analytics): track daily active Discord users

[smmariquit]

Problem

We have no visibility into how many distinct people use the UPLB Tools Discord server per day — useful for growth, volunteer engagement, and correlating with map/editor contribution spikes.

Goal

Track daily active users (DAU) on the Discord server and expose them to maintainers without violating privacy.

Definitions

• Active user (day): unique userId with at least one qualifying event that calendar day (Asia/Manila midnight boundary, unless configured otherwise)
• Qualifying events (v1): sent a message in a text channel, or used a bot slash command, or added a reaction (configurable allowlist)
• Exclude: bots, webhooks, system messages

Acceptance criteria

• Persist daily sets: { date: YYYY-MM-DD, userIds: string[] } (Postgres table or existing bot DB)
• Cron (e.g. 0 16 * * * UTC = 00:00 Manila) finalizes previous day count and stores aggregate { date, dau, uniqueUserIdsCount }
• Privacy: store only user IDs + date; no message content; retention ≤90 days for raw IDs, aggregates kept longer
• Opt-out: respect members with a documented role (e.g. @analytics-opt-out) — excluded from counts
• Maintainer slash command /dau [days] — table or embed: last N days DAU + 7-day average
• Optional weekly cron post to #development or #stats (env DAU_REPORT_CHANNEL_ID): “DAU yesterday: N (7d avg: M)”
• Document in docs/discord-server-guide.md what is collected and retention policy

Implementation notes

• Use events/messageCreate, interactionCreate, messageReactionAdd handlers — debounce per user per day in memory + flush to DB
• Heroku dyno restart: reload today’s partial set from DB on boot
• GDPR-ish hygiene: no export of user IDs to public channels; /dau restricted to maintainer role

Non-goals (v1)

• Per-channel heatmaps
• Cross-linking Discord users to Room TBA editor accounts (separate issue if needed)

References

• discord-bot docs/discord-server-guide.md