feat: ConnectWise PSA integration (#106)

PSA abstraction layer with provider pattern, ConnectWise integration (connection management, ticket linking, note posting, status updates, member mapping), Integrations page UI, Fernet credential encryption, in-memory TTL cache, 6 DB migrations, ConnectWise API reference docs.

backend/app/models/psa_post_log.py

@@ -0,0 +1,58 @@
+"""Audit trail for notes posted to PSA systems."""
+import uuid
+from datetime import datetime, timezone
+from typing import Optional
+
+from sqlalchemy import String, DateTime, Text, ForeignKey
+from sqlalchemy.orm import Mapped, mapped_column, relationship
+from sqlalchemy.dialects.postgresql import UUID
+
+from app.core.database import Base
+
+
+class PsaPostLog(Base):
+    __tablename__ = "psa_post_log"
+
+    id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), primary_key=True, default=uuid.uuid4
+    )
+    session_id: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("sessions.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True,
+    )
+    psa_connection_id: Mapped[Optional[uuid.UUID]] = mapped_column(
+        UUID(as_uuid=True),
+        ForeignKey("psa_connections.id", ondelete="SET NULL"),
+        nullable=True,
+    )
+    ticket_id: Mapped[str] = mapped_column(String(100), nullable=False)
+    note_type: Mapped[str] = mapped_column(String(50), nullable=False)
+    content_posted: Mapped[str] = mapped_column(Text, nullable=False)
+    external_note_id: Mapped[Optional[str]] = mapped_column(
+        String(100), nullable=True
+    )
+    status: Mapped[str] = mapped_column(
+        String(20), nullable=False
+    )  # 'success' or 'failed'
+    error_message: Mapped[Optional[str]] = mapped_column(Text, nullable=True)
+    status_changed_from: Mapped[Optional[str]] = mapped_column(
+        String(100), nullable=True
+    )
+    status_changed_to: Mapped[Optional[str]] = mapped_column(
+        String(100), nullable=True
+    )
+    posted_by: Mapped[uuid.UUID] = mapped_column(
+        UUID(as_uuid=True), ForeignKey("users.id"), nullable=False
+    )
+    posted_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        nullable=False,
+        default=lambda: datetime.now(timezone.utc),
+    )
+
+    # Relationships
+    session = relationship("Session", foreign_keys=[session_id])
+    psa_connection = relationship("PsaConnection", foreign_keys=[psa_connection_id])
+    user = relationship("User", foreign_keys=[posted_by])