Add skill management system: implement Skill and PetSkill models, create SkillController, and update ApplicationDbContext for skills

Controllers/SkillController.cs

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+using Microsoft.AspNetCore.Mvc;
+using PetCompanion.Services;
+
+namespace PetCompanion.Controllers
+{
+    [ApiController]
+    [Route("api/v1/[controller]")]
+    public class SkillController : ControllerBase
+    {
+        private readonly SkillService _skillService;
+        private readonly ILogger<SkillController> _logger;
+        private Guid userId = Guid.Parse("f5f4b3b3-3b7b-4b7b-8b7b-7b7b7b7b7b7b");
+
+        public SkillController(
+            ILogger<SkillController> logger,
+            SkillService skillService)
+        {
+            _logger = logger;
+            _skillService = skillService;
+        }
+
+        [HttpGet]
+        public async Task<IActionResult> GetAvailableSkills()
+        {
+            var skills = await _skillService.GetAvailableSkills();
+            return Ok(skills);
+        }
+
+        [HttpPost("{petId}/allocate/{skillId}")]
+        public async Task<IActionResult> AllocateSkillPoint(string petId, int skillId)
+        {
+            try
+            {
+                var result = await _skillService.AllocateSkillPoint(petId, userId.ToString(), skillId);
+                return Ok(result);
+            }
+            catch (Exception ex)
+            {
+                return BadRequest(ex.Message);
+            }
+        }
+    }
+}