docs: add mkdocs, update links, add architecture documentation

docs/content/adr/0013-database-orm.md

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+# ADR-0013: Database ORM Selection
+
+## Status
+Accepted
+
+## Context
+The platform needs a database ORM/library that:
+- Supports PostgreSQL (primary database)
+- Provides type-safe query building
+- Supports code generation (reduces boilerplate)
+- Handles migrations
+- Supports relationships (many-to-many, etc.)
+- Integrates with Ent (code generation)
+
+Options considered:
+1. **entgo.io/ent** - Code-generated, type-safe ORM
+2. **gorm.io/gorm** - Feature-rich ORM with reflection
+3. **sqlx** - Lightweight wrapper around database/sql
+4. **Standard library database/sql** - No ORM, raw SQL
+
+## Decision
+Use **entgo.io/ent** as the primary ORM for the platform.
+
+**Rationale:**
+- Code generation provides compile-time type safety
+- Excellent schema definition and migration support
+- Strong relationship modeling
+- Good performance (no reflection at runtime)
+- Active development and good documentation
+- Recommended in playbook-golang.md
+- Easy to integrate with OpenTelemetry
+
+## Consequences
+
+### Positive
+- Type-safe queries eliminate runtime errors
+- Schema changes are explicit and versioned
+- Code generation reduces boilerplate
+- Good migration support
+- Strong relationship support
+
+### Negative
+- Requires code generation step (`go generate`)
+- Learning curve for developers unfamiliar with Ent
+- Less flexible than raw SQL for complex queries
+- Generated code must be committed or verified in CI
+
+### Implementation Notes
+- Install: `go get entgo.io/ent/cmd/ent`
+- Initialize schema: `go run entgo.io/ent/cmd/ent init User Role Permission`
+- Use `//go:generate` directives for code generation
+- Run migrations on startup via `client.Schema.Create()`
+- Create wrapper in `internal/infra/database/client.go` for DI injection
+