feat: added hashtag model (#77)

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This PR is related to [this issue](#78)

#### Description
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- Added hashtag model


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- [ ] Bug fix (non-breaking change which fixes an issue)
- [x] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would cause existing
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api/v1/models/__init__.py

@@ -6,3 +6,4 @@
 from api.v1.models.profile_picture import ProfilePicture
 from api.v1.models.social_link import SocialLink
 from api.v1.models.notification import Notification
+from api.v1.models.hashtag import Hashtag

api/v1/models/hashtag.py

@@ -0,0 +1,14 @@
+from sqlalchemy.orm import mapped_column, Mapped
+from sqlalchemy import Integer, String
+
+from api.v1.models.abstract_base import AbstractBaseModel
+
+
+class Hashtag(AbstractBaseModel):
+    __tablename__ = "hashtag"
+
+    tag: Mapped[str] = mapped_column(String(55), nullable=False, unique=True)
+    usage: Mapped[int] = mapped_column(Integer, default=1)
+
+    def __str__(self):
+        return self.tag