feat: publish unblended AI-plan effectiveness series

Report booked cost, entitlement, token coverage, and work as three
series: work per measured token, work per measured+estimated token,
and work per euro. Months below 50% coverage are unfit for token
trends. There is no blended efficiency number.

src/fin_hub/schemas/effectiveness.py

@@ -0,0 +1,34 @@
+"""Read schema for the AI-plan effectiveness surface."""
+
+from __future__ import annotations
+
+from pydantic import BaseModel, ConfigDict
+
+
+class EffectivenessRead(BaseModel):
+    model_config = ConfigDict(extra="forbid")
+
+    period_month: str
+    provider: str
+    provider_account: str | None
+    plan: str
+    currency: str | None
+    booked_amount: str | None
+    entitled_quantity: str | None
+    entitled_unknown: bool
+    plan_label: str | None
+    tokens_measured: int | None
+    tokens_estimated: int | None
+    tokens_unknown: bool
+    coverage: str | None
+    coverage_threshold: str
+    trend_fit: str
+    implied_eur_per_measured_token: str | None
+    implied_eur_per_measured_estimated_token: str | None
+    implied_basis: str
+    work_count: int | None
+    work_unit: str | None
+    work_per_measured_token: str | None
+    work_per_measured_estimated_token: str | None
+    work_per_euro: str | None
+    token_join: str

src/fin_hub/schemas/session_tokens.py

@@ -36,6 +36,14 @@ class SessionTokenSlice(BaseModel):
         return value
 
 
+class SessionTokenWork(BaseModel):
+    model_config = ConfigDict(extra="forbid")
+
+    unit: Literal["task", "outcome"] = "task"
+    count: int = Field(ge=0)
+    source: str = Field(min_length=1)
+
+
 class SessionTokenAssociation(BaseModel):
     model_config = ConfigDict(extra="forbid")
 
@@ -64,6 +72,7 @@ class SessionTokenEvidence(BaseModel):
     source_evidence: list[str] = Field(min_length=1)
     totals: dict[str, SessionTokenSlice] = Field(default_factory=dict)
     associations: list[SessionTokenAssociation] = Field(default_factory=list)
+    work: SessionTokenWork | None = None
 
     @field_validator("totals")
     @classmethod