chore: install larastan at level 6 with accept-all baseline

Installs larastan/larastan ^3.0 (v3.9.6) as a dev-dependency. Level
6 is the starting target — catches missing typehints, method-
existence, null-safety, and model-property existence. Level 8
deferred to a follow-up sprint after level-6 baseline reaches zero.

Baseline error count at install: 1556 errors across 678 analysed
files (41 distinct identifiers).

Top 10 identifiers (errors / files):
  613 /  87  property.notFound
  289 /  52  missingType.generics
  154 /  31  argument.templateType
   98 /  61  missingType.iterableValue
   77 /  32  argument.type
   50 /  26  method.notFound
   35 /  35  method.childReturnType
   32 /   9  method.unresolvableReturnType
   31 /  10  assign.propertyType
   28 /  17  instanceof.alwaysTrue

Composer scripts:
  - composer analyse              — run static analysis
  - composer analyse:baseline     — regenerate baseline
  - composer analyse:clear-cache  — clear PHPStan result cache

Config deviation from plan: checkGenericClassInNonGenericObjectType
was removed in PHPStan 2.x (which Larastan 3 bundles) — setting
dropped from phpstan.neon, otherwise config matches the work
package verbatim. Defaults cover the original intent.

Documentation: /dev-docs/LARASTAN.md added; CLAUDE.md quality-gates
section introduced (with PHPUnit + Pint + Larastan listed).

Backlog: /dev-docs/BACKLOG.md gets 10 per-identifier reduction
sprints (TECH-LARASTAN-01..10) seeded from the actual baseline top
categories, plus TECH-LARASTAN-CI and TECH-LARASTAN-L8 follow-ups.

Memory limit 2G (baseline generation completed within it).

No production behavior change.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

api/composer.json

@@ -19,6 +19,7 @@
     },
     "require-dev": {
         "fakerphp/faker": "^1.23",
+        "larastan/larastan": "^3.0",
         "laravel/pail": "^1.2.2",
         "laravel/pint": "^1.24",
         "laravel/sail": "^1.41",
@@ -55,6 +56,9 @@
             "@php artisan config:clear --ansi",
             "@php artisan test"
         ],
+        "analyse": "vendor/bin/phpstan analyse --memory-limit=2G",
+        "analyse:baseline": "vendor/bin/phpstan analyse --generate-baseline --memory-limit=2G",
+        "analyse:clear-cache": "vendor/bin/phpstan clear-result-cache",
         "post-autoload-dump": [
             "Illuminate\\Foundation\\ComposerScripts::postAutoloadDump",
             "@php artisan package:discover --ansi"