feat(manual-entry): bulk and single entry for the feeds no API serves

Egg, milk and fodder have no usable API for the pilot districts — NECC
publishes HTML, dairy rates exist only as annual state averages, and every
recent Agmarknet fodder row is one market in Ambala. This is how a bank
gets that data in.

Upload stages a PREVIEW batch and writes nothing. It reports what would
land, what was rejected and why, and which rows would overwrite an
existing value. Only commit touches market_signal.

Effective dating: effective_date is the date the fact describes and comes
from the file; observedAt is when we learned it and is stamped at commit.
The uploader cannot reach observedAt — a March rate uploaded in August
must stay invisible to a March scoring run, or the model trains on data
that did not exist yet.

Rejections name themselves: UNKNOWN_DISTRICT, UNKNOWN_COMMODITY,
FUTURE_DATE, BAD_DATE, BAD_NUMBER, UNIT_MISMATCH, PRICE_ORDER,
DUPLICATE_IN_FILE, MISSING_REQUIRED. A price far from last week warns but
is accepted — a genuine price shock is the event this system exists to
catch, and rejecting it would be backwards.

Rows are written with source=MANUAL, which the unique index keeps separate
from a feed's own row for the same week, so manual data never silently
replaces a government feed.

Also distinguishes INSUFFICIENT_HISTORY from NO_SIGNAL in the price
derivations. A newly onboarded manual feed spends its first weeks below
the four-week minimum, and reporting NO_SIGNAL there reads as a broken
integration rather than a series still filling up.

Result: margin_squeeze and output_price_change compute for the first time
— 57 of 142 enterprises, exactly the DAIRY (30) and POULTRY (27) sectors
that have an output commodity.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

pom.xml

@@ -93,6 +93,14 @@
 			<scope>provided</scope>
 		</dependency>
 
+		<!-- Excel upload for manually-sourced signals. poi-ooxml reads .xlsx;
+		     CSV is parsed without a library. -->
+		<dependency>
+			<groupId>org.apache.poi</groupId>
+			<artifactId>poi-ooxml</artifactId>
+			<version>5.3.0</version>
+		</dependency>
+
 		<!-- Testing -->
 		<dependency>
 			<groupId>org.springframework.boot</groupId>