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/.gitignore

@@ -17,6 +17,7 @@
 /public/hot
 /public/storage
 /storage/*.key
+/storage/app/phpstan-tmp
 /storage/pail
 /vendor
 Homestead.json