feat: add optional Headroom compression proxy support

[mydisha]

What

Adds opt-in integration with the Headroom compression proxy. When enabled, KeiRouter sends requests through Headroom for token compression and tracks exact savings (tokens before/after, compression ratio, transforms) alongside existing optimization modes.

Why

Headroom reports exact token counts from its compression proxy (unlike RTK's byte/4 estimate), giving more accurate savings analytics. Integration is fully opt-in so the default make dev / make setup flow remains unchanged.

How

• New package backend/internal/headroom/ — HTTP client that sends OpenAI-format messages to Headroom, applies compressed results back to the canonical core.ChatRequest, and returns Stats (tokens before/after/saved, compression ratio, transforms, CCR hashes). Skips unsupported content parts gracefully.
• Pipeline integration (pipeline.go) — Headroom runs as a compression step in applyTokenSaving. When Headroom.Enabled, it takes precedence over Slimmer; otherwise Slimmer/Terse/Caveman run as before. Failures are non-fatal (logs a warning and continues uncompressed).
• Persistence — Migration 0021_headroom_savings.sql adds headroom_* columns to usage_records; store/models.go and repo_usage.go persist and aggregate headroom stats. Separate from RTK estimates so both systems can be observed independently.
• Observability — meter and observ/metrics track headroom activity and savings; gateway/insights.go and gateway/settings.go expose headroom config and savings in the admin API.
• Dev experience — New make headroom target starts the proxy via Docker or native CLI. KEIROUTER_HEADROOM_AUTO=1 make dev auto-starts it alongside backend + dashboard. compose.yaml adds a headroom service with healthcheck and wires KEIROUTER_HEADROOM__BASE_URL.
• Frontend — Settings page gains headroom controls; Usage page and SavingsBreakdown component surface headroom savings separately from RTK.
• Docs — README and scripts/quickstart.sh updated with headroom setup instructions.

Changed files (21 files, +967/-61)

Area	Files
Backend core	headroom/headroom.go, headroom/headroom_test.go, pipeline/pipeline.go
Storage	store/migrations/0021_headroom_savings.sql, store/models.go, store/repo_usage.go
Observability	meter/meter.go, observ/metrics.go
Gateway	gateway/insights.go, gateway/settings.go, gateway/gemini.go, gateway/handlers.go, app/app.go
Frontend	pages/Settings.tsx, pages/Usage.tsx, components/SavingsBreakdown.tsx, lib/api.ts
Infra/Docs	Makefile, compose.yaml, README.md, scripts/quickstart.sh

Checklist

• make test passes
• make vet passes
• npm run typecheck passes (frontend changed)
• npm run lint — N/A (no lint script configured in frontend/package.json)
• Documentation updated (README, quickstart)
• Config example updated (compose.yaml, Makefile defaults)