fix(config): fail fast on invalid DATABASE_PROVIDER; normalize case (#249)

Address post-merge review findings on the configurable provider
feature. No behavioral change for valid sqlite/postgres values.

- ServiceCollectionExtensions: trim and normalize DATABASE_PROVIDER
  before the switch; add explicit "sqlite"/"" case; throw
  InvalidOperationException for unrecognized values so typos like
  "postgress" no longer silently fall through to SQLite; replace
  null-coalescing guard on DATABASE_URL with IsNullOrWhiteSpace
- entrypoint.sh: normalize DATABASE_PROVIDER to lowercase via tr
  so POSTGRES, Postgres, etc. are handled consistently
- copilot-instructions.md: update Overview to mention both providers;
  replace SQLite-only "Never modify" constraint with a warning about
  ProviderSpecificMigrationsAssembly namespace constants

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: nanotaboada

.github/copilot-instructions.md

@@ -2,7 +2,7 @@
 
 ## Overview
 
-REST API for managing football players built with ASP.NET Core 10. Implements CRUD operations with a layered architecture, EF Core + SQLite persistence, FluentValidation, AutoMapper, and in-memory caching. Part of a cross-language comparison study (Go, Java, Python, Rust, TypeScript). Primarily a learning and reference project — clarity and educational value take precedence over brevity.
+REST API for managing football players built with ASP.NET Core 10. Implements CRUD operations with a layered architecture, EF Core persistence (SQLite by default, PostgreSQL opt-in via `DATABASE_PROVIDER`), FluentValidation, AutoMapper, and in-memory caching. Part of a cross-language comparison study (Go, Java, Python, Rust, TypeScript). Primarily a learning and reference project — clarity and educational value take precedence over brevity.
 
 ## Tech Stack
 
@@ -181,7 +181,7 @@ Example: `feat(api): add player search endpoint (#123)`
 - Production configurations or deployment secrets
 - `.runsettings` coverage thresholds
 - Port configuration (9000)
-- Database type (SQLite — demo/dev only)
+- Migration namespace constants in `ProviderSpecificMigrationsAssembly` — renaming breaks runtime provider filtering for one or both providers
 - CD pipeline tag format (`vX.Y.Z-stadium`) or the stadium name sequence — names are assigned sequentially A→Z from the list in `CHANGELOG.md`; the next name is always the next unused letter
 
 ### Creating Issues

scripts/entrypoint.sh

@@ -10,7 +10,7 @@ log() {
 
 log "✔ Starting container..."
 
-DATABASE_PROVIDER="${DATABASE_PROVIDER:-sqlite}"
+DATABASE_PROVIDER=$(printf '%s' "${DATABASE_PROVIDER:-sqlite}" | tr '[:upper:]' '[:lower:]')
 
 if [ "$DATABASE_PROVIDER" = "postgres" ]; then
     log "✔ Using PostgreSQL database."

src/Dotnet.Samples.AspNetCore.WebApi/Extensions/ServiceCollectionExtensions.cs

@@ -35,20 +35,25 @@ IWebHostEnvironment environment
     {
         services.AddDbContextPool<PlayerDbContext>(options =>
         {
-            var provider = Environment.GetEnvironmentVariable("DATABASE_PROVIDER") ?? "sqlite";
+            var provider = (
+                Environment.GetEnvironmentVariable("DATABASE_PROVIDER") ?? ""
+            )
+                .Trim()
+                .ToLowerInvariant();
 
-            switch (provider.ToLowerInvariant())
+            switch (provider)
             {
                 case "postgres":
-                    var connectionString =
-                        Environment.GetEnvironmentVariable("DATABASE_URL")
-                        ?? throw new InvalidOperationException(
+                    var connectionString = Environment.GetEnvironmentVariable("DATABASE_URL");
+                    if (string.IsNullOrWhiteSpace(connectionString))
+                        throw new InvalidOperationException(
                             "DATABASE_URL is required when DATABASE_PROVIDER=postgres."
                         );
                     options.UseNpgsql(connectionString);
                     break;
 
-                default:
+                case "sqlite":
+                case "":
                     var storagePath = Environment.GetEnvironmentVariable("STORAGE_PATH");
                     var dataSource = !string.IsNullOrWhiteSpace(storagePath)
                         ? storagePath
@@ -61,6 +66,12 @@ IWebHostEnvironment environment
                     }
                     options.UseSqlite($"Data Source={dataSource}");
                     break;
+
+                default:
+                    throw new InvalidOperationException(
+                        $"Unsupported DATABASE_PROVIDER value: '{provider}'. "
+                            + "Valid values are 'sqlite' (default) and 'postgres'."
+                    );
             }
 
             if (environment.IsDevelopment())