fix: resolve remaining 5 QA audit findings (#3, #8, #10, #11, #12)

#3 — Duplicate prospect handling: add_lead now checks (campaign_id, url)
     before insert; bulk_add_leads skips existing URLs.
#8 — Atomic rate limiting: try_increment_* methods atomically check cap
     and increment in a single session; router uses these before send.
#10 — Reply matching via Message-ID: sender generates Message-ID header,
     stored on OutreachAttempt; reply monitor parses In-Reply-To/References;
     poll_replies matches by message_id first, falls back to from_email.
#11 — Save-to-campaign uses existing store results instead of
      re-running expensive deepDiscover.
#12 — Lead status Literal type: Pydantic models enforce valid status
      values; backend validates via LEAD_VALID_STATUSES frozenset;
      frontend API typed as LeadStatus union.

backend/services/backlink_outreach_models.py

@@ -2,6 +2,7 @@ from __future__ import annotations
 
 from pydantic import BaseModel, Field, HttpUrl
 from typing import Dict, List, Optional
+from typing_extensions import Literal
 
 
 class BacklinkKeywordInput(BaseModel):
@@ -93,7 +94,7 @@ class LeadListResponse(BaseModel):
 
 
 class LeadStatusUpdateRequest(BaseModel):
-    status: str = Field(..., min_length=1)
+    status: Literal["discovered", "contacted", "replied", "placed", "bounced", "unsubscribed"]
     notes: Optional[str] = None
     campaign_id: Optional[str] = Field(default=None, min_length=1)
 
@@ -329,7 +330,7 @@ class ConversionFunnelResponse(BaseModel):
 
 class BulkStatusUpdateRequest(BaseModel):
     lead_ids: List[str] = Field(..., min_length=1)
-    status: str = Field(..., min_length=1)
+    status: Literal["discovered", "contacted", "replied", "placed", "bounced", "unsubscribed"]
     notes: Optional[str] = None
     campaign_id: Optional[str] = Field(default=None, min_length=1)