feat(api): optimistic-concurrency contract for replace-all PUTs — PR1 infra + Block reference (#253)

Adds the shared optimistic-concurrency contract so a stale second tab can no longer
silently overwrite a fresher edit. PR1 lands the infra + the Block reference aggregate;
PRs 2–4 fan the same recipe across the other 8 roots (design: #253#issuecomment-8472).

Contract
- `IVersionedAggregate` (`int Version`) on all 9 replace-all roots (ProgramSchedule,
  Block, Template, DecoTemplate, Playlist, Collection, Playout, MultiCollection,
  RerunCollection), EF-mapped `.IsConcurrencyToken()`; one dual-provider migration
  `AddAggregateVersions` (nullable:false, default 0).
- Strong `ETag` of `Version` on the aggregate GET; `If-Match` on the PUT; mismatch →
  412 (distinct from the §3a 409 build-lock guard). Successful PUT returns the new ETag.
- `PreconditionFailedError : BaseError` → 412 in `ApiResults.ToErrorResult`;
  `ConcurrencyHeaders.ParseIfMatch/SetETag`; malformed If-Match → 400; `*`/absent =
  Phase-1 force-write.

Block reference wiring
- Handler: standalone `Either` via `CheckVersion` AFTER validation (never through
  `Apply`, which Join()-flattens the subtype to 422), unconditional `Version++`,
  `SaveChangesWithConcurrencyGuard` backstop (DbUpdateConcurrencyException → 412).
- `BlockViewModel.Version` (header-only, not echoed in the body); controller sets the
  ETag on GET items and on the successful PUT.
- SPA: `client.requestWithMeta` seam; `blocks.getBlockItemsWithMeta` + `replaceBlock`
  If-Match/ETag round-trip; `BlockEditor` holds the ETag, sends If-Match, and on 412
  opens a blocking "changed elsewhere — reload" dialog.

Tests
- Handler contract tests: stale-If-Match → 412 (no mutation), matching/absent → success
  + bump, no-op save still bumps, and a two-context racing save → 412; proven
  non-vacuous (drop `.IsConcurrencyToken()` → the race test fails).
- Controller tests: malformed If-Match → 400, If-Match threaded to the command, ETag on
  GET/PUT, 412 passthrough. SPA: requestWithMeta ETag, replaceBlock If-Match, 412 dialog.

Docs: api-conventions §7a, spa-conventions §4a, domain-model glossary, decisions log.

Refs #253
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ErsatzTV/wwwroot/openapi/v1.json

@@ -565,7 +565,7 @@
           "Blocks"
         ],
         "summary": "Replace a block and its items",
-        "description": "Replaces the block's name/minutes/stop-scheduling and its full item list. Item indexes are assigned from the array order. Minutes must be greater than zero, divisible by 5, and at most 24 hours.",
+        "description": "Replaces the block's name/minutes/stop-scheduling and its full item list. Item indexes are assigned from the array order. Minutes must be greater than zero, divisible by 5, and at most 24 hours. Send the ETag from the items GET as If-Match to reject a stale overwrite with 412 (issue #253); a successful response carries the new ETag.",
         "parameters": [
           {
             "name": "id",
@@ -623,6 +623,26 @@
               }
             }
           },
+          "400": {
+            "description": "Bad Request",
+            "content": {
+              "text/plain": {
+                "schema": {
+                  "$ref": "#/components/schemas/ProblemDetails"
+                }
+              },
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/ProblemDetails"
+                }
+              },
+              "text/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/ProblemDetails"
+                }
+              }
+            }
+          },
           "404": {
             "description": "Not Found",
             "content": {
@@ -643,6 +663,26 @@
               }
             }
           },
+          "412": {
+            "description": "Precondition Failed",
+            "content": {
+              "text/plain": {
+                "schema": {
+                  "$ref": "#/components/schemas/ProblemDetails"
+                }
+              },
+              "application/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/ProblemDetails"
+                }
+              },
+              "text/json": {
+                "schema": {
+                  "$ref": "#/components/schemas/ProblemDetails"
+                }
+              }
+            }
+          },
           "422": {
             "description": "Unprocessable Entity",
             "content": {
@@ -672,6 +712,7 @@
           "Blocks"
         ],
         "summary": "Get block items",
+        "description": "Returns the block's items and a strong ETag of the block's version. Pass that ETag back as If-Match on the replace (PUT) to detect a concurrent edit (issue #253).",
         "parameters": [
           {
             "name": "id",