feat: Redis multi-container support (Phase 1.6)

[adamy]

Summary

Phase 1.6 makes BotWire correct behind a load balancer. New opt-in BotWire.Redis package moves conversation sessions and rate-limit counters out of process memory into Redis, so they're shared across instances and survive restarts. Defaults are unchanged — without .AddBotWireRedis(...) everything behaves exactly as v0.2.0.

builder.Services.AddBotWire(opts => { /* ... */ })
                .AddBotWireRedis("localhost:6379");

What's included

• IRateLimitStore abstraction in BotWire.Core — no StackExchange.Redis dependency in Core.
• RedisConversationStore — JSON sessions, sliding SessionTtl, dual-history round-trip, MaxHistoryMessages cap enforced identically to the in-memory store.
• RedisRateLimitStore — atomic INCRBY + conditional EXPIRE (Lua); TTL clamped ≥1s so day-boundary token-budget keys aren't deleted.
• RateLimiter async variants use the store when present; per-minute path waits for the fixed window to roll rather than admitting over-quota turns. MaxConcurrentSessions stays per-container by design.
• RedisShop sample — ASP.NET API (Redis sessions + distributed limits + email escalation) + Vite/React/TS shopfront with a custom chatbox on headless botwire-js.
• Docs: v0.3.0 release notes, README Samples section, per-package NuGet readme.

Testing

• New Core tests: distributed per-minute (within/over quota), shared token budget. (254 pass)
• New Redis tests (skip if no Redis): session/dual-history/flags round-trip, cross-instance sharing, TTL refresh, history-cap parity, sub-second-TTL key survival. (19 pass against Docker Redis)
• Existing AspNetCore (27) + full solution build, 0 warnings.

Notes

• Three findings from /code-review high were fixed in this branch (per-minute enforcement, history-cap parity, TTL clamp) with regression tests.
• VersionPrefix left at 0.2.0 — version bump + v0.3.0 tag is the separate release cut.

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