Group batch-sync mappings by EC ID before KV updates

CHANGELOG.md

@@ -9,6 +9,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Changed
 
+- The S2S `/_ts/api/v1/batch-sync` endpoint now validates the full batch and calls the CAS-protected update path once per distinct normalized EC ID. The last valid UID wins within a group, and infrastructure failures reject the failing and each unprocessed group, so accepted and `kv_unavailable` input indexes may interleave.
 - **Breaking:** Auction providers and bidder routes now use the configuration-first `[auction.providers.<id>]` and `[auction.bidders.<id>]` maps. The removed `[auction].providers = [...]` list and removed server fields under `[integrations.prebid]` and `[integrations.aps]` are rejected even when those integrations are disabled, and `ts config push` rejects the old shape before publication. Move PBS `server_url` to provider `endpoint`, server timeout to provider `timeout_ms`, request controls and bidder-parameter overrides to the `prebid-server` `profile_config`, notification suppression to `notifications`, and each former server bidder to an `[auction.bidders.<id>]` route. Move APS endpoint, timeout, account, inventory, debug, and creative controls to an `aps` provider and its `profile_config`. Browser Prebid settings remain under `[integrations.prebid]`; values such as timeout and debug that previously affected both browser and server behavior must now be configured for each owner. Provider endpoints must be absolute HTTPS URLs. Only bidder codes present in `[auction.bidders]` are folded into Trusted Server requests; unlisted publisher bids remain native browser demand. Provider response names now use the configured provider ID, such as `pbs-main`, instead of the legacy literal `prebid`; audit consumers that match `AuctionResponse.provider`. This schema has no mixed-version-safe deployment order: old binaries reject the maps and new binaries reject the retired fields, so activate the new binary and config blob together. Rollbacks must restore an old-schema blob together with the old binary.
 - **Breaking** — Admin Basic-auth coverage now includes `GET /_ts/admin/ec`, `GET /_ts/admin/ec/{id}`, and `GET /_ts/admin/eids`. Existing configurations whose `[[handlers]]` patterns protect only the key-management endpoints now fail startup; broaden coverage before deploying, preferably with a namespace-boundary pattern such as `^/_ts/admin(?:/|$)`. Coverage of the dynamic `/_ts/admin/ec/{id}` route is no longer inferred from ID-shaped samples: the router accepts any segment after `/_ts/admin/ec/` and Basic Auth runs on the raw path before routing, so patterns anchored to the EC ID grammar (for example `^/_ts/admin/ec/[a-f0-9]{64}[.][A-Za-z0-9]{6}$`) are rejected in favor of a prefix-level matcher. Placeholder and well-known weak handler passwords (`changeme`, `password`, `admin`, `replace-with-…`) now fail startup on every handler rather than only on handlers inferred to cover an admin endpoint, because first-match-wins handler selection lets a narrow handler shadow the admin namespace.
 - Prebid Server provider endpoints now normalize origin-only legacy `server_url` values to `/openrtb2/auction`. Query parameters are preserved, the canonical path loses a trailing slash, and configured non-root custom paths remain exact.