fix(docker): bump builder to Go 1.26, download sqlc binary, fix test-runner Go version

The builder stage was previously bumped to Go 1.26 but the test-runner stage
remained on Go 1.25, causing 'go.mod requires go >= 1.26.0' errors during
test execution.

Go 1.26 introduced stricter validation of replace directives in dependency
go.mod files, which broke 'go install sqlc@latest' (sqlc v1.31.0 has replace
directives). Replace go install with a direct binary download from GitHub
releases, matching the existing kepubify download pattern.

Bump test-runner from golang:1.25-alpine to golang:1.26-alpine to match the
go.mod requirement.

Dockerfile

@@ -1,5 +1,5 @@
 # Build stage
-FROM golang:1.25-alpine AS builder
+FROM golang:1.26-alpine AS builder
 
 WORKDIR /app
 
@@ -7,8 +7,10 @@ WORKDIR /app
 RUN apk add --no-cache nodejs npm curl git
 
 # Install Go tools (cached well)
+RUN wget -O /tmp/sqlc.tar.gz https://github.com/sqlc-dev/sqlc/releases/download/v1.31.0/sqlc_1.31.0_linux_amd64.tar.gz && \
+    tar -xzf /tmp/sqlc.tar.gz -C /usr/local/bin sqlc && \
+    rm /tmp/sqlc.tar.gz
 RUN --mount=type=cache,target=/root/go/pkg/mod \
-  go install github.com/sqlc-dev/sqlc/cmd/sqlc@latest && \
   go install github.com/a-h/templ/cmd/templ@latest
 
 # Copy package files and install npm dependencies (cached unless package.json changes)
@@ -38,7 +40,7 @@ RUN --mount=type=cache,target=/root/go/pkg/mod \
 
 # Test runner stage - includes Go runtime and test dependencies
 # This stage is ONLY used for running tests, never deployed to production
-FROM golang:1.25-alpine AS test-runner
+FROM golang:1.26-alpine AS test-runner
 
 RUN apk --no-cache add ca-certificates curl