feat: FluxAI multi-step autonomy + rate limiting + image pipeline

Two production-grade hardening additions and one cost optimisation.

FLUXAI AUTONOMY RESTORED (api/chat)
- Brings back the multi-step agentic flow that the system prompt was
  always designed for. The "temporarily removed maxSteps" comment is
  gone — replaced with the AI SDK 6 equivalent stopWhen: stepCountIs(5).
- Cap at 5 chained tool calls per turn bounds latency + LLM cost.
- maxDuration raised 30s → 60s to absorb tool-chain runs.
- Result: one user prompt now triggers, e.g. search_installations →
  energy_savings_calculator → show_case_study → schedule_consultation
  in a single turn — exactly the SPIN methodology in the prompt.

RATE LIMITING (src/lib/rateLimit.ts + api/chat)
- Token-bucket per IP: 30 messages burst, sustained 30/minute. Trips
  to 429 with Retry-After + X-RateLimit-Remaining headers when abused.
- IP extracted from x-forwarded-for (Nginx already passes this).
- In-memory Map with 10-min GC of stale buckets — no Redis dep.
  If we scale to multiple replicas later, swap the Map for Upstash.
- Protects the OpenAI quota from someone hammering the chat endpoint.

IMAGE PIPELINE (src/lib/imageOptimizer.ts)
- sharp-based optimizer: auto-orient (EXIF), cap at 2560px long side,
  re-encode WebP@85, content-hash filename. Re-uploads with same
  content reuse the same hash; new content gets a new URL — perfect
  cache invalidation without header tricks.
- Opt-in via optimize=1 form/query param on /api/assets POST.
- Hero CMS and Site Settings uploads turn it on automatically (those
  are user-facing brand assets where compression matters most).
- App/news/parts uploads remain untouched (editors may be uploading
  CAD drawings, datasheets, etc. that shouldn't be transcoded).
- Falls back gracefully to a no-op for unsupported formats (SVG, GIF,
  videos, anything sharp can't decode) so it never breaks an upload.

DOCKERFILE
- Adds vips/vips-dev for sharp on Alpine + --include=optional so the
  @img/sharp-linuxmusl-x64 prebuilt is downloaded
- Explicitly copies node_modules/sharp + node_modules/@img to the
  runner stage (Next.js trace can miss conditional deps).

NO DB SCHEMA CHANGES.

package.json

@@ -30,6 +30,7 @@
     "react": "19.2.4",
     "react-dom": "19.2.4",
     "resend": "^6.9.3",
+    "sharp": "^0.34.5",
     "speakeasy": "^2.0.0",
     "tailwind-merge": "^3.5.0",
     "three": "^0.183.2",