aep-api: DB model rates + write-time USD stamping + org usage endpoint (BE for #291)

[axewilledge]

BE handshake for #291 (Usage & cost v2). Supersedes #249 / PR #269 (closed unmerged; capture pipeline to be resurrected from aep-api/usage-capture-249).

Requested backend work (aep-api)

1. Resurrect the capture pipeline from aep-api/usage-capture-249 — turn-level capture (agents usage: frames → agent_turns token columns), execution-level capture (runner AEP_USAGE log line → JobWatcher parse → executions token columns), unchanged.

2. model_rates table (new) — model_id PK, input_per_mtok / output_per_mtok / cache_read_per_mtok / cache_write_per_mtok (USD), updated_at. Seeded by migration with the platform's current model at current Anthropic rates. Ops-managed (SQL); no CRUD API this round. Replaces MODEL_PRICING_MODEL_ID / MODEL_RATE_*_PER_MTOK env config and config.ModelPricingConfig.

3. Write-time USD stamping — at capture time, resolve the record's model_id against model_rates and persist cost_usd on the same row (agent_turns.cost_usd, executions.cost_usd). Unknown model or missing rate row → cost_usd NULL (never guessed). No read-time pricing: platform/modelcost is retired. Historical rows are immutable — a later rate change affects only subsequent stamps. No backfill of pre-v2 rows (decided on #291).

Proposed contract diff

• New GET /usage/projects (org-scoped): { projects: [{ projectId, displayName, deleted: bool, costUsd: number|null, inputTokens, outputTokens, cacheReadTokens, cacheWriteTokens }] } — one row per project having any usage, including deleted projects (identified from usage rows' stored project slug).
• Remove GET /projects/{p}/usage (the v1 per-phase shape; its only consumer — the v1 chips — is being removed).

Rationale

Read-time derivation reprices history whenever a rate changes; costs must be immutable once incurred. Rates-as-data removes single-model config coupling. Full context + decisions: #291.

[axewilledge]

Backend PR is open: #301 (based on the #291 FE branch console/usage-page-291 so the diff is BE-only; retarget to aep-rewrite once #300 merges).

Implements everything in the handshake: model_rates table + seed, boot-loaded write-time Stamper, cost_usd stamped on agent_turns + executions, and GET /usage/projects folding both sources. Capture pipeline resurrected from the closed #269 branch onto the new contract. Verified live against local dev — endpoint returns exact token sums with null cost on pre-stamping rows (no-backfill). Details + the two verification caveats are on the PR.