Add fediverse social tables, protocols, and implementations

6 new ORM models (remote actors, following, remote posts, local posts,
interactions, notifications), 20 new FederationService methods with
SQL implementations and stubs, WebFinger client, and Alembic migration.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

utils/webfinger.py

@@ -0,0 +1,68 @@
+"""WebFinger client for resolving remote AP actor profiles."""
+from __future__ import annotations
+
+import logging
+
+import httpx
+
+log = logging.getLogger(__name__)
+
+AP_CONTENT_TYPE = "application/activity+json"
+
+
+async def resolve_actor(acct: str) -> dict | None:
+    """Resolve user@domain to actor JSON via WebFinger + actor fetch.
+
+    Args:
+        acct: Handle in the form ``user@domain`` (no leading ``@``).
+
+    Returns:
+        Actor JSON-LD dict, or None if resolution fails.
+    """
+    acct = acct.lstrip("@")
+    if "@" not in acct:
+        return None
+
+    _, domain = acct.rsplit("@", 1)
+    webfinger_url = f"https://{domain}/.well-known/webfinger"
+
+    try:
+        async with httpx.AsyncClient(timeout=10, follow_redirects=True) as client:
+            # Step 1: WebFinger lookup
+            resp = await client.get(
+                webfinger_url,
+                params={"resource": f"acct:{acct}"},
+                headers={"Accept": "application/jrd+json, application/json"},
+            )
+            if resp.status_code != 200:
+                log.debug("WebFinger %s returned %d", webfinger_url, resp.status_code)
+                return None
+
+            data = resp.json()
+            # Find self link with AP content type
+            actor_url = None
+            for link in data.get("links", []):
+                if link.get("rel") == "self" and link.get("type") in (
+                    AP_CONTENT_TYPE,
+                    "application/ld+json; profile=\"https://www.w3.org/ns/activitystreams\"",
+                ):
+                    actor_url = link.get("href")
+                    break
+
+            if not actor_url:
+                log.debug("No AP self link in WebFinger response for %s", acct)
+                return None
+
+            # Step 2: Fetch actor JSON
+            resp = await client.get(
+                actor_url,
+                headers={"Accept": AP_CONTENT_TYPE},
+            )
+            if resp.status_code == 200:
+                return resp.json()
+
+            log.debug("Actor fetch %s returned %d", actor_url, resp.status_code)
+    except Exception:
+        log.exception("WebFinger resolution failed for %s", acct)
+
+    return None