Add ROADMAP.md for infrastructure phases 2-4

[diese-tech]

Summary

Issue #5 ("Design multi-server scaling architecture for 100+ guilds") was just closed as completed — all its acceptance criteria are met by the current Phase 1 (single worker + SQLite) implementation. But that issue's comment thread also contains the phased plan for what comes after Phase 1: Postgres migration, sharding, and a dashboard/API, each with specific trigger conditions. Once an issue is closed, that plan effectively goes stale/hard-to-find.

This adds ROADMAP.md at the repo root as the version-controlled home for that forward-looking plan, so it survives independently of any one issue and gets updated via normal PRs instead of issue-comment archaeology.

Contents

• Phase 1 (complete) — summary of what Issue Design multi-server scaling architecture for 100+ guilds #5 asked for and what shipped.
• Phase 2 — Postgres migration — the exact trigger conditions and rationale from the original issue discussion (more than one worker, public invite beyond beta, dashboard/API writes, production SLA, or guild count beyond controlled beta).
• Phase 3 — Sharding — explicitly deferred, "do not shard prematurely," same as the original issue said.
• Phase 4 — Dashboard/API — not started, not currently planned.
• Smaller open items — two loose threads surfaced during Phase 1 development that don't block anything but shouldn't get lost: the unimplemented inherit_category_permissions/enabled profile columns from the original schema proposal, and the still-undecided architecture for wiring the landing page's placeholder stats to live data.
• History — a short dated log of the issue's opening, closing, and this document's creation.

Test plan

Documentation-only change — no code, schema, or CI-relevant files touched.

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