feat(step-3.6): cloud provider integrations (Gmail, Outlook, Teams)

- Add app/integrations/__init__.py: Fernet token encryption helpers,
  EmailMessage/ChatMessage dataclasses, get_provider() factory
- Add app/integrations/gmail.py: GmailClient with async fetch_messages(),
  token refresh, configurable label/sender/date filters
- Add app/integrations/ms_graph.py: MSGraphClient with fetch_emails()
  (Outlook) and fetch_messages() (Teams), MSAL token refresh, OData filters
- Update app/core/agent_runner.py: replace run_cloud_agent() stub with
  full 8-step implementation; extend _finalize_run() for cloud config type
- Update app/config/settings.py: add OAuth + Fernet encryption settings
- Update requirements.txt: google-api-python-client, google-auth-*,
  msal, cryptography
- Add tests/test_integrations.py: 47 tests covering all integration code
- Update tests/test_agent_runner.py: replace stub test with 7 real tests

All 76 new/updated tests pass.

app/integrations/__init__.py

@@ -0,0 +1,164 @@
+"""Cloud provider integration utilities.
+
+Provides:
+  * Shared message dataclasses (``EmailMessage``, ``ChatMessage``) used by
+    both the Gmail and MS Graph clients and consumed by ``agent_runner``.
+  * ``get_provider()`` — factory that returns the correct client given a
+    provider name and decrypted OAuth credentials dict.
+  * ``encrypt_token()`` / ``decrypt_token()`` — Fernet-based at-rest
+    encryption for OAuth tokens stored in ``cloud_agent_configs``.
+
+Encryption rationale
+--------------------
+Unlike user content (which is E2E-encrypted client-side and **never**
+decrypted server-side), OAuth tokens *must* be decrypted server-side
+because the backend makes provider API calls on behalf of the user.
+The Fernet key lives solely in ``OAUTH_ENCRYPTION_KEY`` env var — it
+is never returned to clients.
+"""
+
+from __future__ import annotations
+
+import json
+import logging
+from dataclasses import dataclass, field
+from datetime import datetime
+from typing import TYPE_CHECKING
+
+from cryptography.fernet import Fernet, InvalidToken
+
+from app.config.settings import settings
+
+if TYPE_CHECKING:
+    from app.integrations.gmail import GmailClient
+    from app.integrations.ms_graph import MSGraphClient
+
+logger = logging.getLogger(__name__)
+
+# ── Shared message types ──────────────────────────────────────────────────
+
+
+@dataclass
+class EmailMessage:
+    """A single email message fetched from Gmail or Outlook."""
+
+    id: str
+    subject: str
+    sender: str
+    body_text: str
+    date: datetime
+    labels: list[str] = field(default_factory=list)
+
+    @property
+    def as_text(self) -> str:
+        """Return a human-readable text representation for LLM extraction."""
+        date_str = self.date.strftime("%Y-%m-%d %H:%M")
+        labels_str = f" [{', '.join(self.labels)}]" if self.labels else ""
+        return (
+            f"From: {self.sender}\n"
+            f"Date: {date_str}{labels_str}\n"
+            f"Subject: {self.subject}\n\n"
+            f"{self.body_text}"
+        )
+
+
+@dataclass
+class ChatMessage:
+    """A single Teams chat or channel message fetched from MS Graph."""
+
+    id: str
+    content: str
+    sender: str
+    channel: str | None
+    date: datetime
+
+    @property
+    def as_text(self) -> str:
+        """Return a human-readable text representation for LLM extraction."""
+        date_str = self.date.strftime("%Y-%m-%d %H:%M")
+        channel_str = f" [channel: {self.channel}]" if self.channel else ""
+        return (
+            f"From: {self.sender}\n"
+            f"Date: {date_str}{channel_str}\n\n"
+            f"{self.content}"
+        )
+
+
+# ── Fernet helpers ────────────────────────────────────────────────────────
+
+
+def _get_fernet() -> Fernet:
+    """Return a ``Fernet`` instance using ``settings.OAUTH_ENCRYPTION_KEY``.
+
+    Raises ``RuntimeError`` if ``OAUTH_ENCRYPTION_KEY`` is not set — callers
+    must ensure this is configured before persisting OAuth tokens.
+    """
+    key = settings.OAUTH_ENCRYPTION_KEY
+    if not key:
+        raise RuntimeError(
+            "OAUTH_ENCRYPTION_KEY is not set. "
+            "Generate one with: python -c \"from cryptography.fernet import Fernet; print(Fernet.generate_key().decode())\""
+        )
+    return Fernet(key.encode() if isinstance(key, str) else key)
+
+
+def encrypt_token(token_info: dict) -> str:
+    """Fernet-encrypt an OAuth credential dict and return a base64 string.
+
+    Stores the full ``{access_token, refresh_token, token_uri, client_id,
+    client_secret, scopes, expiry}`` dict (or equivalent MSAL shape).
+
+    Raises:
+        RuntimeError: OAUTH_ENCRYPTION_KEY is not configured.
+        ValueError: ``token_info`` is not a non-empty dict.
+    """
+    if not isinstance(token_info, dict) or not token_info:
+        raise ValueError("token_info must be a non-empty dict")
+    plaintext = json.dumps(token_info).encode("utf-8")
+    return _get_fernet().encrypt(plaintext).decode("utf-8")
+
+
+def decrypt_token(encrypted: str) -> dict:
+    """Decrypt a Fernet-encrypted token string and return the credential dict.
+
+    Raises:
+        RuntimeError: OAUTH_ENCRYPTION_KEY is not configured.
+        ValueError: The encrypted string is invalid or was encrypted with a
+            different key.
+    """
+    try:
+        plaintext = _get_fernet().decrypt(encrypted.encode("utf-8"))
+        return json.loads(plaintext)
+    except (InvalidToken, json.JSONDecodeError) as exc:
+        raise ValueError(f"Failed to decrypt OAuth token: {exc}") from exc
+
+
+# ── Provider factory ──────────────────────────────────────────────────────
+
+
+def get_provider(
+    provider: str,
+    credentials_info: dict,
+) -> "GmailClient | MSGraphClient":
+    """Return the correct provider client for *provider*.
+
+    Parameters
+    ----------
+    provider:
+        One of ``"gmail"``, ``"outlook"``, ``"teams"``.
+    credentials_info:
+        Decrypted OAuth credential dict (Google or Microsoft shape).
+
+    Raises:
+        ValueError: Unknown provider name.
+    """
+    if provider == "gmail":
+        from app.integrations.gmail import GmailClient
+        return GmailClient(credentials_info)
+    if provider in {"outlook", "teams"}:
+        from app.integrations.ms_graph import MSGraphClient
+        return MSGraphClient(credentials_info)
+    raise ValueError(
+        f"Unknown cloud provider {provider!r}. "
+        "Supported: 'gmail', 'outlook', 'teams'."
+    )