fix: Phase 2-3 DB isolation + Gate 3 data layer real connection (AGENTS.md v16.0)

**DB Isolation (P0):**
- Test connection string: kartselldb → kartselldb_test (prevents accidental production truncates)
- Production Host appsettings unchanged (kartselldb is correct for operations)

**Gate 3 Data Layer (P1):**
- Remove StubKrxDataService from ModelOperationsModule DI
- Register real KrxDataService as typed HttpClient in Program.cs
- KrxDataService already has built-in fallback to stub data when KRX_API_KEY is missing
- No behavior change for local dev (key missing → stub data); production ready (key present → real API)

**Tech Debt Registration (AGENTS.md no undocumented magic):**
- DEBT-009: PBO/Sharpe calculation simplified (needs proper CSCV methodology)
- DEBT-010: Model prediction uses fixed quantities (needs real position-sizing)
- DEBT-011: Cost 2x simulation uses linear formula (needs full re-simulation)
- DEBT-012: False-exit analysis unimplemented (always returns 0)
- DEBT-013: Plaintext DB password in appsettings.json (security debt)
- DEBT-014: Duplicate/reconciliation detection placeholders (infrastructure debt)

Gate 3 marked "rehearsal ready" (real KRX data, simplified analytics).
See TECH_DEBT_REGISTER.md for full impact/effort estimates.

Co-Authored-By: Claude Haiku 4.5 <noreply@anthropic.com>

tests/KArtSell.Integration.Tests/appsettings.Development.json

@@ -1,5 +1,5 @@
 {
   "ConnectionStrings": {
-    "Postgres": "Host=127.0.0.1;Port=5432;Database=kartselldb;Username=kartsell;Password=kartsell4321@!"
+    "Postgres": "Host=127.0.0.1;Port=5432;Database=kartselldb_test;Username=kartsell;Password=kartsell4321@!"
   }
 }