docs(architecture): v0.3 — document implemented backend (API surface, points rules, DB schema, roadmap)

[argszero]

Summary

Docs-only update: bump docs/architecture.md from v0.2 → v0.3 to reflect the implemented backend reality (P0-A/B/C + P1, now at v0.3.0). The doc previously only described the planned architecture (v0.2, 2026-08-15) — before the backend existed. This brings it in sync with the code:

• §4.1 Gateway — marked implemented: OpenAI Chat Completions + Anthropic Messages dual protocol, SSE streaming (streaming usage metering, client-disconnect no-settle), available-balance precheck (gift + permanent), plus a full API surface list (healthz / auth login / api-keys / gateway / models / sharings / wallet / transactions / dashboard / admin credits·users·usage)
• §4.2 Key pool — upstream key AES-256-GCM encryption at rest (v1:<nonce>:<cipher>; ATP_MASTER_KEY → config.server.master_key → dev-random fallback; startup plaintext migration)
• §4.3/4.4 Metering & Ledger — points formula, transactional settle (90/10 share, all-or-nothing rollback), dual-balance account (balance permanent + gift_balance), daily gift (1pt/day × 10 days, same-day expiry, lazy expiry cleanup), gift-first deduction (earliest expiry), admin top-up → topup ledger
• §4.6 Admin — admin API implemented (role check via AuthUser.role, 403, seed accounts)
• §5 DB design — replaced with actual table list (incl. gift_grants, schema_version, quotas dual columns)
• §6 Deployment — config file layout + run command
• §7 Roadmap — P0 (v0.1.0→v0.2.2, PR feat(backend): P0-A skeleton — axum server, TOML config, SQLite schema+seed, argon2 auth, api-key endpoints (v0.2.0) #71–feat(gateway): P0-C SSE 流式转发 + 上游 key 加密 + 共享/钱包/交易 API (v0.2.2) #74) and P1 (v0.3.0, PR feat(points): P1 points rules — daily gift/expiry/gift-first deduction/admin top-up #75) marked complete

Verification

• Facts cross-checked against source: src/routes/mod.rs (routes), src/db.rs (schema v3), src/billing.rs / src/gift.rs (points rules), src/crypto.rs (AES-256-GCM), Cargo.toml (aes-gcm 0.10)
• No code changes — docs only