finalize Go + SQLite stack decisions for Synology NAS deployment

DEV_PORTAL_M02_WEB_STACK.md

@@ -1,8 +1,44 @@
 # Milestone 2: Web Stack Selection
 
-**Status**: Planning
+**Status**: Decided
 **Goal**: Choose backend technologies for the developer portal and app store API.
 
+## Decision
+
+**Go** with the following stack:
+
+```
+Language:   Go 1.22+
+Framework:  Chi (lightweight, idiomatic)
+Database:   SQLite via modernc.org/sqlite (pure Go, no CGO)
+Migrations: golang-migrate
+Validation: go-playground/validator
+Auth:       Custom JWT + OAuth2
+Deployment: Single Docker container on Synology NAS
+```
+
+### Rationale
+
+1. **NAS-friendly** - Tiny Docker image (~15MB), low RAM (~30-50MB)
+2. **Cross-compilation** - Easy build for ARM64 or AMD64 Synology models
+3. **Pure Go SQLite** - `modernc.org/sqlite` requires no CGO, cross-compiles easily
+4. **Single container** - Go binary + SQLite + Litestream in one image
+5. **Standard library** - HTTP, JSON, crypto all built-in
+
+### Target Deployment
+
+```
+Synology NAS (Docker)
+├── mosis-portal container (~15MB image)
+│   ├── Go binary
+│   ├── SQLite database (WAL mode)
+│   └── Litestream backup
+└── Volumes
+    ├── /volume1/mosis/data/portal.db
+    ├── /volume1/mosis/backups/
+    └── /volume1/mosis/packages/
+```
+
 ---
 
 ## Overview
@@ -308,12 +344,10 @@ Based on:
 
 ## Deliverables
 
-- [ ] Prototype API in Go
-- [ ] Prototype API in Node.js (if needed)
-- [ ] Benchmark comparison document
-- [ ] Final selection with rationale
+- [x] Final selection with rationale
 - [ ] Project structure template
 - [ ] Development environment setup guide
+- [ ] Base Go project scaffolding
 
 ---