feat: RSS embeddings, semantic news in chat, article-to-chat, richer briefings

- Embed RSS items at fetch time (nomic-embed-text); backfill at startup
- Semantic news search injected into chat system prompt ("Recent News You've Seen")
  when items match query above 0.55 cosine threshold (independent of note RAG)
- "Discuss in chat" button on news cards — creates a seeded conversation with
  the article title + full content, navigates directly to the new chat
- Briefing compilation now passes 500-char article excerpts (not just headlines)
  to the LLM and uses 8192 num_ctx to accommodate the larger prompt

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

alembic/versions/0035_add_rss_item_embeddings.py

@@ -0,0 +1,28 @@
+"""Add rss_item_embeddings table for semantic news search."""
+
+import sqlalchemy as sa
+from alembic import op
+from sqlalchemy.dialects.postgresql import JSONB
+
+revision = "0035"
+down_revision = "0034"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.create_table(
+        "rss_item_embeddings",
+        sa.Column("rss_item_id", sa.Integer(), sa.ForeignKey("rss_items.id", ondelete="CASCADE"), primary_key=True),
+        sa.Column("user_id", sa.Integer(), nullable=False),
+        sa.Column("embedding", JSONB(), nullable=False),
+        sa.Column("updated_at", sa.DateTime(timezone=True), nullable=False, server_default=sa.func.now()),
+    )
+    op.create_index("ix_rss_item_embeddings_user_id", "rss_item_embeddings", ["user_id"])
+    op.create_index("ix_rss_item_embeddings_rss_item_id", "rss_item_embeddings", ["rss_item_id"])
+
+
+def downgrade() -> None:
+    op.drop_index("ix_rss_item_embeddings_rss_item_id", table_name="rss_item_embeddings")
+    op.drop_index("ix_rss_item_embeddings_user_id", table_name="rss_item_embeddings")
+    op.drop_table("rss_item_embeddings")