Trust only the upstream X-Gateway-Model header in the AI gateway

An upstream the gateway forwards to may itself emit X-Gateway-* headers
(e.g. another DevPlace-style gateway), which can collide with the ones
about to be built for the response. Only X-Gateway-Model is ever trusted
from upstream and relayed as-is - it is the one field an upstream can
legitimately know better than we do (it may have resolved an alias or
served a different pinned version). Every other header (cost, tokens,
latency, context, app-reference) is always our own measurement and is
never overwritten, since blending in an upstream's own accounting would
corrupt the usage ledger's per-model rollups and the quota math built on
top of it.

usage.upstream_reported_model() extracts that one header defensively
(case-insensitive lookup, rejects anything oversized or containing a
control character) and gateway._apply_served_model() applies it, display-
only, at the tail of every response-header build across chat, streaming,
embeddings, images, and passthrough.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BWJy6PrMMt5hwWxQwia2rd

pyproject.toml

@@ -1,6 +1,6 @@
 [project]
 name = "devplacepy"
-version = "1.0.4"
+version = "1.0.5"
 description = "DevPlace - The Developer Social Network"
 requires-python = ">=3.12"
 dependencies = [