Declare and create the calls(system_id, started_at) index dedup needs

Dedup was failing on essentially every inbound call in production. dedup.py
queries system_id == X with a started_at range; that composite index was
neither declared in firestore.indexes.json nor present in the live c2-server
database, so the query returned FAILED_PRECONDITION, dedup swallowed it as a
warning, and every duplicate check degraded to "not a duplicate".

While AI is off that only cost duplicate call documents. With a window open it
would have paid Whisper and Gemini twice for every double-heard transmission,
and fed Gate B5's cost measurement a figure that is wrong for a reason
unrelated to the pipeline being measured. Two documents for one transmission is
also the exact input shape that produces a spurious second incident.

The index is created on c2-server and building. This declares it in source so
the file and the live database agree.

Refs server-26#84, #33, #45.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

infra/firestore/firestore.indexes.json

@@ -30,6 +30,15 @@
         { "fieldPath": "ended_at", "order": "ASCENDING" }
       ]
     },
+    {
+      "//": "c2-core internal/dedup.py:84 — system_id == X AND started_at within a +/- window. Was live-failing on essentially every inbound call (server-26#84): dedup caught the FAILED_PRECONDITION and degraded to \"not a duplicate\", so double-heard transmissions were stored twice and would have been transcribed and correlated twice the moment an AI window opened. Created directly on c2-server 2026-08-28.",
+      "collectionGroup": "calls",
+      "queryScope": "COLLECTION",
+      "fields": [
+        { "fieldPath": "system_id", "order": "ASCENDING" },
+        { "fieldPath": "started_at", "order": "ASCENDING" }
+      ]
+    },
     {
       "//": "c2-core internal/vocabulary_learner.py:290 — system_id == X AND ended_at >= cutoff. Backend only; was live but undeclared until 2026-08-23.",
       "collectionGroup": "calls",