fix: push limit/offset into the query instead of materializing then slicing

cog-api's paginate() sliced an already-fully-materialized result: every
page of a deep sweep re-ran the whole cog_spending()/cog_revenue() query
and re-listified every row, just to keep up to 1000 and discard the
rest. A 193,105-row/194-page fleet-wide sweep (cog_explorer's Southern
guide, corpus summary build) repeated that full cost 194 times and
wedged the production server for hours on 2026-08-06 -- single request,
CPU-bound, single-threaded plumber process, no other request could get
through, not even /health.

cog_spending()/cog_revenue() gain optional limit/offset, pushed into
.build_verb_sql() as SQL LIMIT/OFFSET behind the existing (already
deterministic) ORDER BY. The full unpaginated row count rides along via
COUNT(*) OVER() in the same scan -- exposed as a total_rows attribute --
so a caller walking pages never needs a second round trip to ask how
many there are. A page now costs O(limit), not O(full result).

Mutually exclusive with complete = TRUE (which fills a grid over the
FULL requested (year, category) space -- pagination over a partial slice
of already-grouped rows has no defined meaning for the cells it would
fill) and with recipe (whose result comes from a separate, not-yet-wired
query path). Both abort with a clear classed condition rather than
silently ignoring the parameter.

limit/offset default to NULL; every existing call site is unaffected.

R/revenue.R

@@ -54,7 +54,7 @@ cog_revenue <- function(govid, years, category = NULL,
                         per_capita = FALSE, adjust_to_year = NULL,
                         basis = c("harmonized", "raw"), recipe = NULL,
                         revenue_concept = c("general", "total"),
-                        complete = FALSE) {
+                        complete = FALSE, limit = NULL, offset = NULL) {
   # flow_prefixes no longer classifies rows (crosswalk revenue_subtype
   # membership does -- General Revenue, i.e. everything except
   # insurance_trust) -- it only scopes the recipe-suggestion machinery to
@@ -73,6 +73,8 @@ cog_revenue <- function(govid, years, category = NULL,
     basis          = basis,
     recipe         = recipe,
     revenue_concept = revenue_concept,
-    complete       = complete
+    complete       = complete,
+    limit          = limit,
+    offset         = offset
   )
 }