Add LinkedIn OAuth connect with portal-saved app credentials.

Replace manual token paste with Authorization Code flow; store Client ID/secret in the DB from the social accounts UI and surface a copyable callback URL for LinkedIn Auth setup.

site/social/models.py

@@ -10,6 +10,29 @@ class Platform(models.TextChoices):
     LINKEDIN = "linkedin", "LinkedIn"
 
 
+class SocialAppCredentials(UUIDPrimaryKeyModel, TimeStampedModel):
+    """
+    Developer-app OAuth credentials entered in the portal (not env).
+
+    client_secret is Fernet-encrypted. One row per platform.
+    """
+
+    platform = models.CharField(max_length=16, choices=Platform.choices, unique=True)
+    client_id = models.CharField(max_length=255, blank=True)
+    encrypted_client_secret = models.TextField(blank=True)
+
+    class Meta:
+        verbose_name_plural = "social app credentials"
+        ordering = ["platform"]
+
+    def __str__(self) -> str:
+        return f"{self.platform} app credentials"
+
+    @property
+    def is_configured(self) -> bool:
+        return bool(self.client_id and self.encrypted_client_secret)
+
+
 class SocialAccount(UUIDPrimaryKeyModel, TimeStampedModel):
     platform = models.CharField(max_length=16, choices=Platform.choices)
     label = models.CharField(max_length=120)