spending: prefill annual £ from actualbudget trailing 12mo

Adds a thin read-only client for the actualbudget HTTP API
(`fire_planner/actualbudget.py`) and a `GET /spending/annual` endpoint
that returns trailing-N-month spending broken out by category group.

Default exclusions ("Investments and Savings", "Budget Reset") strip
out wealth transfers so the headline number reflects actual
consumption — for Viktor's data, ~£41k/yr instead of the raw £210k
total. Caller can pass `?exclude=...` to override.

Frontend uses the headline `total_gbp` to autofill the Annual spending
input (same pattern as nw_seed from networth), with a small
provenance line below the input showing the window + which groups
were excluded.

Auth: 3 new env vars (ACTUALBUDGET_API_URL/KEY/SYNC_ID) sourced from
Vault `secret/fire-planner` via the existing ExternalSecret —
infra/stacks/fire-planner applied separately. Backend silently keeps
the hardcoded default if the upstream is unreachable.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

fire_planner/api/schemas.py

@@ -133,6 +133,35 @@ class NetWorthHistory(BaseModel):
     points: list[NetWorthHistoryPoint]
 
 
+# ── annual spending (from actualbudget) ──────────────────────────────
+
+
+class SpendingMonth(BaseModel):
+    """One month's outflows (positive £) by category group, after
+    income groups have been dropped upstream."""
+    month: str  # "YYYY-MM"
+    by_group: dict[str, Decimal]
+    total_gbp: Decimal
+
+
+class AnnualSpending(BaseModel):
+    """Aggregated trailing-N-month spending pulled from actualbudget.
+
+    `total_gbp` is what we suggest as the "Annual spending" default in
+    the WhatIf form: the sum of all category groups across the trailing
+    window, *minus* whichever groups the caller asked to exclude
+    (default: investments/savings transfers, which aren't consumption).
+    """
+    months: int
+    window_start: str  # "YYYY-MM" (oldest month included)
+    window_end: str    # "YYYY-MM" (newest)
+    excluded_groups: list[str]
+    total_gbp: Decimal               # the headline number
+    raw_total_gbp: Decimal           # before exclusions, for transparency
+    by_group_total_gbp: dict[str, Decimal]  # 12-mo group sums (incl. excluded)
+    monthly: list[SpendingMonth]
+
+
 # ── life events ──────────────────────────────────────────────────────