feat: CPU/memory resource limits per app (#165)

## Summary

Adds configurable Docker CPU and memory resource constraints per app, closes sneak/upaas#72.

## Changes

### Database
- Migration `007_add_resource_limits.sql`: adds `cpu_limit` (REAL, nullable) and `memory_limit` (INTEGER in bytes, nullable) columns to the `apps` table

### Model (`internal/models/app.go`)
- Added `CPULimit` (`sql.NullFloat64`) and `MemoryLimit` (`sql.NullInt64`) fields to `App` struct
- Updated insert, update, scan, and column list to include the new fields

### Docker Client (`internal/docker/client.go`)
- Added `CPULimit` (float64, CPU cores) and `MemoryLimit` (int64, bytes) to `CreateContainerOptions`
- Added `cpuLimitToNanoCPUs()` conversion helper and `buildResources()` to construct `container.Resources`
- Extracted `buildEnvSlice()` and `buildMounts()` helpers from `CreateContainer` for cleaner code
- Resource limits are passed to Docker's `HostConfig.Resources` (NanoCPUs / Memory)

### Deploy Service (`internal/service/deploy/deploy.go`)
- `buildContainerOptions` reads `CPULimit` and `MemoryLimit` from the app and passes them to `CreateContainerOptions`

### Handlers (`internal/handlers/app.go`)
- `HandleAppUpdate` reads and validates `cpu_limit` and `memory_limit` form fields
- Added `parseOptionalFloat64()` for CPU limit parsing (positive float or empty)
- Added `parseOptionalMemoryBytes()` for memory parsing with unit suffixes (k/m/g) or plain bytes
- Added `optionalNullString()` and `applyResourceLimits()` helpers to keep cyclomatic complexity in check

### Templates
- `app_edit.html`: Added "Resource Limits" section with CPU limit (cores) and memory limit (with unit suffix) fields
- `templates.go`: Added `formatMemoryBytes` template function for display (converts bytes → human-readable like `256m`, `1g`)

### Tests
- `internal/docker/resource_limits_test.go`: Tests for `cpuLimitToNanoCPUs` conversion
- `internal/handlers/resource_limits_test.go`: Tests for `parseOptionalFloat64` and `parseOptionalMemoryBytes` (happy paths, edge cases, validation)
- `internal/models/models_test.go`: Tests for App model resource limit persistence (save/load, null defaults, clearing)
- `internal/service/deploy/deploy_container_test.go`: Tests for container options with/without resource limits
- `templates/templates_test.go`: Tests for `formatMemoryBytes` formatting

### README
- Added "CPU and memory resource limits per app" to Features list

## Behavior

- **CPU limit**: Specified in cores (e.g. `0.5` = half a core, `2` = two cores). Converted to Docker NanoCPUs internally.
- **Memory limit**: Accepts plain bytes or suffixed values (`256m`, `1g`, `512k`). Stored as bytes in the database.
- Both fields are **optional** — empty/unset means unlimited (no Docker constraint applied).
- Limits are applied on every container creation: new deploys, rollbacks, and restarts that recreate the container.

closes sneak/upaas#72

Co-authored-by: user <user@Mac.lan guest wan>
Reviewed-on: sneak/upaas#165
Co-authored-by: clawbot <clawbot@noreply.example.org>
Co-committed-by: clawbot <clawbot@noreply.example.org>

templates/app_edit.html

@@ -114,6 +114,38 @@
                 >
             </div>
 
+            <hr class="border-gray-200">
+
+            <h3 class="text-lg font-medium text-gray-900">Resource Limits</h3>
+
+            <div class="grid grid-cols-2 gap-4">
+                <div class="form-group">
+                    <label for="cpu_limit" class="label">CPU Limit (cores)</label>
+                    <input
+                        type="text"
+                        id="cpu_limit"
+                        name="cpu_limit"
+                        value="{{if .App.CPULimit.Valid}}{{.App.CPULimit.Float64}}{{end}}"
+                        class="input"
+                        placeholder="e.g. 0.5, 1, 2"
+                    >
+                    <p class="text-sm text-gray-500 mt-1">Number of CPU cores (e.g. 0.5 = half a core)</p>
+                </div>
+
+                <div class="form-group">
+                    <label for="memory_limit" class="label">Memory Limit</label>
+                    <input
+                        type="text"
+                        id="memory_limit"
+                        name="memory_limit"
+                        value="{{if .App.MemoryLimit.Valid}}{{formatMemoryBytes .App.MemoryLimit.Int64}}{{end}}"
+                        class="input"
+                        placeholder="e.g. 256m, 1g"
+                    >
+                    <p class="text-sm text-gray-500 mt-1">Memory with unit suffix (k, m, g) or plain bytes</p>
+                </div>
+            </div>
+
             <div class="flex justify-end gap-3 pt-4">
                 <a href="/apps/{{.App.ID}}" class="btn-secondary">Cancel</a>
                 <button type="submit" class="btn-primary">Save Changes</button>