feat: daily budget throttling with rate-aware proxy sleep

[BYK]

Summary

Add a configurable daily USD spending cap to the Lore gateway. When the budget is approached, the gateway applies an invisible proxy-level sleep before forwarding requests upstream — progressively slowing agents rather than hard-blocking them.

How It Works

1. Set budget via the UI dashboard (/ui/costs) or env var LORE_DAILY_BUDGET. Persisted in SQLite kv_meta table — survives restarts, easy to tune.

2. Rate detection: Cost-rate EMA (α=0.15) tracks spending velocity across all sessions. Time-gap-adjusted alpha means long idle gaps naturally decay the EMA.

3. Throttle curve: 60s × pressure² × tanh(overshoot/3) — smooth, C∞ continuous. Starts at sub-second delays at 60% budget, maxes at 60s under extreme overshoot. No cliff edges.

4. Invisible proxy sleep: The agent doesn't know it's being throttled — the upstream just appears to take a little longer. A 3-5s sleep is imperceptible during a normal coding turn.

5. Cache TTL safety cap: Sleep is capped at 50% of remaining cache TTL window to prevent cache busts that would increase costs.

Throttle Behavior

Spend %	Rate vs Target	Delay
<50%	any	0s (no throttle)
60%	2×	~0.8s
70%	2×	~3.1s
80%	2×	~6.9s
80%	5×	~19.2s
95%	3×	~36.9s

Configuration

• UI: Set/disable budget from the Costs page (/ui/costs)
• Env var: LORE_DAILY_BUDGET=10.00 (overrides DB value)
• Resolution: env var > DB > 0 (disabled)
• Default: 0 (disabled, zero overhead on hot path)

Dashboard

• Global: Budget progress bar with spend/budget ratio, current rate, throttle event count
• Per-session: Throttle event count + total delay when throttled

What's NOT Throttled

• Meta requests (title generation, etc.) — cheap
• Compaction requests — intercepted locally
• Worker calls — their cost IS counted, but they're not delayed (have their own rate limiting)

Files Changed

File	Changes
packages/gateway/src/cost-tracker.ts	Daily spend accumulator, cost-rate EMA, throttle curve, DB-backed budget, pre-request cost estimator
packages/gateway/src/pipeline.ts	Throttle interception before forwardToUpstream() with cache TTL safety cap
packages/gateway/src/server.ts	Bootstrap daily spend from DB on startup
packages/gateway/src/ui.ts	Budget settings form, progress bar, throttle diagnostics
packages/gateway/test/budget-throttle.test.ts	28 tests covering all throttle zones, monotonicity, smoothness, EMA, accumulation

No new files (except test). No DB schema changes. No core package changes.