chore: bump Go toolchain to 1.26 (TASK-763) (#247)

* chore: bump Go toolchain to 1.26 (TASK-763)

Bump Go from 1.25 to 1.26 across all toolchain pins:

- go.mod — go 1.25.0 → go 1.26.0
- Dockerfile — golang:1.25-alpine → golang:1.26-alpine
- .github/workflows/ci.yml — three setup-go steps (Go, Go-Postgres, E2E jobs)
- .github/workflows/release.yml — release pipeline

No `toolchain` directive: the repo is pre-launch with no external
contributors yet, so we set the floor where we want it (hard requirement).

Verified locally before commit:
- golangci-lint v2.11.4 builds and runs under Go 1.26.2 (pinned in CI)
- golang:1.26-alpine and 1.26.2-alpine images present on Docker Hub
- go build ./... clean
- go vet ./... clean
- go test ./... all pass

Parent: PLAN-644 (OSS Repo Hygiene and Launch Polish).

* chore: gofmt -w under Go 1.26 (TASK-763)

Apply Go 1.26's gofmt to the codebase. ~41 files reformatted, all
struct-tag whitespace realignment — no semantic changes. Verified:

- gofmt -l ./cmd ./internal returns empty after
- go build ./... still clean
- go test ./... still passes (run before commit)

Bundling the gofmt diff with the toolchain bump in the same PR because
the formatting drift is a direct consequence of moving from 1.25 to
1.26; splitting them creates a mandatory two-PR ordering for no value.

Parent: PLAN-644.

* docs: bump documented Go floor to 1.26 (TASK-763)

Match go.mod's hard 1.26.0 requirement in the source-build instructions.
Caught by Codex review round 1 on PR #247.

- README.md:158 — "Go 1.25+" → "Go 1.26+"
- CONTRIBUTING.md:9 — "Go 1.25+" → "Go 1.26+"

.github/workflows/ci.yml

@@ -30,7 +30,7 @@ jobs:
 
       - uses: actions/setup-go@40f1582b2485089dde7abd97c1529aa768e1baff # v5.6.0
         with:
-          go-version: "1.25"
+          go-version: "1.26"
 
       - name: Create web build placeholder for embed
         run: mkdir -p web/build && echo "placeholder" > web/build/.gitkeep
@@ -99,7 +99,7 @@ jobs:
 
       - uses: actions/setup-go@40f1582b2485089dde7abd97c1529aa768e1baff # v5.6.0
         with:
-          go-version: "1.25"
+          go-version: "1.26"
 
       - name: Create web build placeholder for embed
         run: mkdir -p web/build && echo "placeholder" > web/build/.gitkeep
@@ -157,7 +157,7 @@ jobs:
 
       - uses: actions/setup-go@40f1582b2485089dde7abd97c1529aa768e1baff # v5.6.0
         with:
-          go-version: "1.25"
+          go-version: "1.26"
 
       - uses: actions/setup-node@49933ea5288caeca8642d1e84afbd3f7d6820020 # v4.4.0
         with:

.github/workflows/release.yml

@@ -34,7 +34,7 @@ jobs:
 
       - uses: actions/setup-go@40f1582b2485089dde7abd97c1529aa768e1baff # v5.6.0
         with:
-          go-version: "1.25"
+          go-version: "1.26"
 
       - uses: actions/setup-node@49933ea5288caeca8642d1e84afbd3f7d6820020 # v4.4.0
         with: